doi.org/10.26434/chemrxiv.14219450.v1

Uncovering Halide Mixing and Octahedral Dynamics in Vacancy-OrderedDouble Perovskites by Multinuclear Magnetic Resonance SpectroscopyAbhoy Karmakar, Srija Mukhopadhyay, Pierre Gachod, Arturo Gomez, Guy Bernard, Alex Brown, VladimirMichaelis

Submitted date: 15/03/2021 • Posted date: 16/03/2021Licence: CC BY-NC-ND 4.0Citation information: Karmakar, Abhoy; Mukhopadhyay, Srija; Gachod, Pierre; Gomez, Arturo; Bernard, Guy;Brown, Alex; et al. (2021): Uncovering Halide Mixing and Octahedral Dynamics in Vacancy-Ordered DoublePerovskites by Multinuclear Magnetic Resonance Spectroscopy. ChemRxiv. Preprint.https://doi.org/10.26434/chemrxiv.14219450.v1

Vacancy-ordered double perovskites Cs2SnX6 (X = Cl, Br, I) have emerged as promising lead-free andambient-stable materials for photovoltaic and optoelectronic applications. To advance these promisingmaterials, it is crucial to determine the correlations between physical properties and their local structure anddynamics. Solid-state NMR spectroscopy of multiple NMR-active nuclei (133Cs, 119Sn and 35Cl) in thesecesium tin(IV) halides has been used to decode the structure, which plays a key role in the materials’ opticalproperties. The 119Sn NMR chemical shifts span approximately 4000 ppm and the 119Sn spin-latticerelaxation times span three orders of magnitude when the halogen goes from chlorine to iodine in thesediamagnetic compounds. Moreover, ultrawideline 35Cl NMR spectroscopy for Cs2SnCl6 indicates an axiallysymmetric chlorine electric field gradient tensor with a large quadrupolar coupling constant of ca. 32 MHz,suggesting a chlorine that is directly attached to Sn(IV) ions. Variable temperature 119Sn spin lattice relaxationtime measurements uncover the presence of hidden dynamics of octahedral SnI6 units in Cs2SnI6 with a lowactivation energy barrier of 12.45 kJ/mol (0.129 eV). We further show that complete mixed-halide solidsolutions of Cs2SnClxBr6−x and Cs2SnBrxI6−x (0 ≤ x ≤ 6) form at any halogen compositional ratio. 119Sn and133Cs NMR spectroscopy resolve the unique local SnClnBr6−nand SnBrnI6−n (n = 0−6) octahedral andCsBrmI12−m (m = 0−12) cuboctahedral environments in the mixed-halide samples. The experimentallyobserved 119Sn NMR results are consistent with magnetic shielding parameters obtained by densityfunctional theory computations to verify random halogen distribution in mixed-halide analogues. Finally, wedemonstrate the difference in the local structures and optical absorption properties of Cs2SnI6 samplesprepared by solvent-assisted and solvent-free synthesis routes.

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Uncovering Halide Mixing and Octahedral Dynamics in Vacancy-Or-dered Double Perovskites by Multinuclear Magnetic Resonance Spectroscopy AbhoyKarmakar,SrijaMukhopadhyay,PierreG.B.Gachod,VictorArturoMora-Gomez,GuyM.Bernard,AlexBrown,andVladimirK.Michaelis*

DepartmentofChemistry,UniversityofAlberta,Edmonton,AlbertaT6G2G2,Canada

ABSTRACT:Vacancy-ordereddoubleperovskitesCs2SnX6(X=Cl,Br,I)haveemergedaspromisinglead-freeandambient-stablematerialsforphotovoltaicandoptoelectronicapplications.Toadvancethesepromisingmaterials,itiscrucialtodeter-minethecorrelationsbetweenphysicalpropertiesandtheirlocalstructureanddynamics.Solid-stateNMRspectroscopyofmultipleNMR-activenuclei(133Cs,119Snand35Cl)inthesecesiumtin(IV)halideshasbeenusedtodecodethestructure,whichplaysakeyroleinthematerials’opticalproperties.The119SnNMRchemicalshiftsspanapproximately4000ppmandthe119Snspin-latticerelaxationtimesspanthreeordersofmagnitudewhenthehalogengoesfromchlorinetoiodineinthesediamagneticcompounds.Moreover,ultrawideline35ClNMRspectroscopyforCs2SnCl6indicatesanaxiallysymmetricchlorineelectricfieldgradienttensorwithalargequadrupolarcouplingconstantofca.32MHz,suggestingachlorinethatisdirectlyattachedtoSn(IV)ions.Variabletemperature119SnspinlatticerelaxationtimemeasurementsuncoverthepresenceofhiddendynamicsofoctahedralSnI6unitsinCs2SnI6withalowactivationenergybarrierof12.45kJ/mol(0.129eV).Wefurthershowthatcompletemixed-halidesolidsolutionsofCs2SnClxBr6−xandCs2SnBrxI6−x(0≤x≤6)formatanyhalogencompositionalratio.119Snand133CsNMRspectroscopyresolvetheuniquelocalSnClnBr6−nandSnBrnI6−n(n=0−6)octahedralandCsBrmI12−m(m=0−12)cuboctahedralenvironmentsinthemixed-halidesamples.Theexperimentallyobserved119SnNMRresultsareconsistentwithmagneticshieldingparametersobtainedbydensityfunctionaltheorycomputationstoverifyrandomhalogendistributioninmixed-halideanalogues.Finally,wedemonstratethedifferenceinthelocalstructuresandopticalabsorptionpropertiesofCs2SnI6samplespreparedbysolvent-assistedandsolvent-freesynthesisroutes.

INTRODUCTION Lead halide perovskites (APbX3; A = Cs+, CH3NH3+,CH(NH2)2+;X=Cl−,Br−, I−)havegainedtremendousatten-tionassemiconductingmaterialsduetotheirdesirableop-ticalandelectricalpropertiesincludinginphotovoltaicso-larcells,1 light-emittingdiodes,2hard-radiationdetectors,3photocatalyticreactions4andbeyond.5,6Althoughlead-con-tainingperovskiteshavehighpowerconversionefficiencies(PCE)of25.5% inphotovoltaic solar cells,7 they,unfortu-nately,sufferfrommediocrechemicalstability8–11andthepotentialriskof leadtoxicity.12,13Thetin(II)halideperov-skitesASnX3 arepossiblealternativeswith lower toxicity,buttheyexhibitlowerPCEofupto10%andrapidlyoxidizeinair(Sn2+toSn4+).14–17Vacancy-ordered double perovskites A2BX6 (A = Cs+,CH3NH3+,CH(NH2)2+;B=Sn4+;X=Cl−,Br−,I−)areanalterna-tivefamilyofperovskites,demonstratingmuchhigheram-bientstabilityandlowertoxicity.18ThestructureofA2BX6canbedescribedasanABX3perovskite,withhalfoftheB-sitesremovedinanorderedfashion(Figure1a).19Forex-ample,Cs2SnX6arecubicinstructurewith12halogenssur-roundingtheA-sitetoformaCsX12cuboctahedron,whereas

theB-siteisoctahedrallycoordinatedwithsixhalogenani-onstoformBX6octahedra(Figure1a).Theiodideanalogue,Cs2SnI6,hasabandgap(1.25−1.62eV)closetotheoptimumvalue (1.34 eV) to maximize thin-film solar cell effi-ciency.18,20,21Recently,Cs2SnI6hasshownpromisingoptoe-lectronicpropertiestodevelopair-stableandlead-freeper-ovskite photovoltaics.22 The mixed-halide analoguesCs2SnBrxI6−xrevealbandgaptailorabilitywithhalogencom-position,23makingthemsuitableashole-transportingmate-rialsinsolid-statedye-sensitizedsolarcells.24Ontheotherhand, Cs2SnX6 nanocrystals have demonstrated tunablelight emission and photocatalytic activity.25–27 Recently,doped(Bi3+andSb3+)Cs2SnCl6hasbeenshowntoproduceefficientblueandorange-redlightemission;thisdevelop-mentbringsaboutdenovoopportunitiesforthesemateri-alsassolid-statelightingcandidates.28–30To date, vacancy-ordered halide double perovskites havebeenextensivelycharacterizedbyX-raydiffraction (XRD)techniques,whichprovide information about the averagelong-rangestructure,butnotaboutthelocalchemicalenvi-ronments. Fundamentally, it is essential to correlate thebulk photophysical properties with the microstructuralcharacteristicsforperovskites.Solid-statenuclearmagnetic

resonance(NMR)spectroscopyisarobustanalyticalchar-acterizationtool todetermineshort- (<5Å)andmedium-(5−10Å)rangestructuresaswellasiondynamicsinperov-skites.16,31–47Morespecifically,133Cs(I=7/2,Qm=−0.34fm2,100%abundance)48isanidealNMR-activenucleustoiden-tifythechemicalenvironmentsaroundtheA-siteinperov-skites.36,42,47,49–52119Sn(I=1/2,8.6%abundance),themostreceptive nucleus among three NMR-active tin isotopes(115Sn,117Sn,119Sn)(TableS1),48hasbeenusedtoresolvethelocalB-sitestructuralenvironmentsandhalogendynamicsin ABX3 perovskites and other tin-containing com-pounds.16,17,53–59 Furthermore, the 119Sn NMR spectra dis-playsanextensiverangetodiamagneticSn-containingcom-poundswhichhasbeen recently extended to spannearly6,000ppm.16,60–62Thisstudyshowsanimpressiveabilitytotailortheopticalbandgapovera3eVrangeusingahighenergymechano-chemicalsyntheticdesignofvacancyordereddoubleperov-skiteCs2SnClxBr6−xandCs2SnBrxI6−xmixed-halidematerials.PowderXRDandNMRspectroscopyallowidentificationofthelong-,medium-andshort-rangechemicalandcoordina-tionenvironmentsaroundCsandSnsitesuponinterhalo-gen substitutions in mixed-halide analogues. The experi-mentally observed broad (>100 kHz) and multiple reso-nancesinthe119SnNMRspectraforthemixed-halideana-loguesarecorrelatedwith119Snmagneticshieldingparam-etersobtainedfromdensity functionaltheory(DFT)com-putations and reveal the importance of the spin-orbitshielding term.Variable temperature 119SnNMR capturestheactivationenergyoftherapidSnI6octahedraldynamicsinCs2SnI6.Finally,Cs2SnI6materialspreparedbysolvent-as-sistedorsolvent-freesynthesisroutesarediscussedtoun-covertheinfluencesofsolvent-vs.solvent-freesynthesisonthelocalchemicalstructureandbulkopticalproperties.Thecomparisonisimportantasmechanochemistrycontinuestobeshownasaneffectivegreenerapproachforsynthesizingsolids.

Figure1.(a)CrystalstructureofCs2SnX6,localCsX12cubocta-hedral environment of 12 halides around cesium, and localSnX6octahedralenvironmentofsixhalidesaroundtin.(b)Op-ticalabsorptionspectra(inset–photographs)and(c)PowderXRDpatternsofsolventsynthesizedCs2SnX6parents.

Table1.Unitcellparameters,directbandgap,solid-state133Csand119SnNMRresults(νr=13kHz,B0=11.75T)forsolventsynthesizedCs2SnX6parents.

Samples Cellparameter(Å)

Bandgap(eV)

133CsNMR 119SnNMR

δ(ppm) fwhm(Hz) δ(ppm) fwhm(kHz) T1(s)

Cs2SnCl6 10.3826(1) 4.48 125.8±0.1 62±2 –708±1 1.5±0.1 146±6Cs2SnBr6 10.8377(2) 3.13 112.3±0.2 175±5 –1964±2 5.2±0.2 5.4±0.2

Cs2SnI6 11.6342(2) 1.42 –46.1±0.1 66±2 –4540±5 11.5±0.5 0.170±0.007

Figure2.Solid-state(a)133Cs,(b)119SnNMRspectraand(c)inversionrecoveryplottomeasure119Snspin-lattice(T1)relaxationvaluesforthesolventsynthesizedCs2SnX6parents.NMRexperimentswereperformedat11.75Twithmagic-anglespinningfre-quencyof13kHz.

RESULTS AND DISCUSSION SolventSynthesizedCs2SnX6Parents.Cs2SnX6(X=Cl−,Br−,I−) parents were prepared by solvent-assisted syntheticroutesstartingwithCsXandSnX4precursors(seeSI).Asde-termined by energy-dispersive X-ray spectroscopy (EDS),the chemical compositions agree well with the expectedCs2SnX6chemicalformulationformedbysolventsynthesis(TableS2andFigureS1).Cs2SnX6samplesarepolycrystal-line with micron-sized particles as determined by field-emission scanning electron microscopy (FESEM) (FigureS2).ThecolorofCs2SnX6changesfromwhite(Cs2SnCl6)tofaintyellow(Cs2SnBr6)toblack(Cs2SnI6),consistentwithadecreaseintheiropticalabsorptionedge(Figure1b).ThepowderXRDpatternsareconsistentwithacubicstructure(K2PtCl6-type,spacegroupFm3"m)forallthreeCs2SnX6par-entsatroomtemperature(Figure1c).Thecellparameters,asrefinedbyLeBailfittingofthepowderXRDpatterns,sys-tematicallyincreasefortheCs2SnX6parentsastheionicra-diusoftheassociatedhalogenanionincreasesfrom1.81År(Cl–)<1.96År(Br–)<2.20År(I–)(Table1).63The optical absorption spectra,which are extracted fromthediffusereflectancespectrabytheKubelka-Munkfunc-tion,exhibitagradualshiftoftheabsorptionedgetolowerenergyasClisreplacedbyBrandfollowedbyIinCs2SnX6parents. Considering a direct bandgap property for thesesemiconductingmaterials,64theopticalbandgapvaluesareextractedfromthelinearregionintheTaucplotof(αhν)2againstenergy(FigureS3).ThebandgapinCs2SnX6parentsexhibits adramaticdecrease from4.48eV forCs2SnCl6to1.42eVforCs2SnI6,spanningacross3eV(Table1).Solid-state133Csand119SnNMRspectroscopywasusedtoinvestigatethelocalhalogenenvironmentsaroundcesiumand tin.TheCsatomsoccupya crystallographic sitewith4"3msymmetryinCs2SnX6.Twelvenearest-neighboringhal-ogenatomssurroundthemtoformalocalCsX12cuboctahe-dralenvironment(Figure1a).Accordingly,the133Csmagic-angle spinning (MAS)NMR spectra exhibit a single sharpresonancewithchemicalshiftsof125.8ppm(fullwidthathalf maximum, fwhm = 62 Hz) for Cs2SnCl6, 114.6 ppm(fwhm=175Hz)forCs2SnBr6and−41.4ppm(fwhm=66Hz) forCs2SnI6, indicatingoneunique crystallographicCssite(Figure2aandTable1).Agradualchangeinthe133CsNMR chemical shift towards lower frequency is in agree-mentwitharecentobservationonCsSnX3perovskites,16butin contrast to CsPbX3 perovskites,36 where an oppositetrendinthechemicalshiftwasobserved.DuetoCsresidinginthecenterofaperfectCsX12cuboctahedralenvironmentinCs2SnX6parents,avanishing 133Csquadrupolecouplingconstant (CQ = 0 kHz) is expectedwith no spinning side-bands(SSBs)underMASconditions.However,alow-inten-sity(<1%)SSBwasobservedunderslowMAS(νr=5kHz)forallthreeCs2SnX6parents(FigureS4),whichisassociatedtointrinsicdefectssuchashalogenvacancies.49,65

The Sn atoms occupym3"m site symmetry in the Cs2SnX6structure with six nearest-neighbor halogen atoms sur-roundingSntoformlocalSnX6octahedralunits(Figure1a).The Cs2SnX6 parents exhibit symmetric 119Sn NMR reso-nanceswithnoevidenceofSSBsunderMASconditions(Fig-ure2b),consistentwiththehighlysymmetriclocalSnX6oc-tahedral environment. The 119Sn NMR spectra in Cs2SnX6shifttowardslowerfrequencywithachemicalshiftof−708ppmforCs2SnCl6,−1964ppmforCs2SnBr6and−4540ppmforCs2SnI6,alargechangespanningover3800ppmfordia-magnetictin-containingcompounds.ThetrendofincreasedSn shielding (to lower frequency)with increasedhalogenatomicnumberobservedhere,anormalhalogendepend-ence(NHD),isoppositetopreviousstudiesonSn(II)-basedperovskites(ASnX3),whereaninversehalogendependence(IHD)wasobservedwithasmallerchemicalshiftrange(<1300ppm).16,56,66Theunprecedentedchangein119Snchem-icalshiftwasfurtherstudiedusingtheoreticalcalculations,which revealed that the diamagnetic and paramagneticshieldingtermsarenearlyunchanged.Hence,thechangesinchemicalshiftoriginatefromconsiderablespin-orbitef-fectswherebythistermchangesca.4500ppmasthehalideanionsincreaseinsize(TableS3),emphasizingtheneedtoconsiderrelativisticandspin-orbiteffectswhenperformingDFTcalculationsforthesematerialswhichcontainheavyel-ements.The119SnNMRlinewidthsdependonthehalogeninCs2SnX6,increasingmonotonicallyfrom1.5(Cs2SnCl6)to5.2(Cs2SnBr6) to 11.5 kHz (Cs2SnI6). For Cs2SnBr6, the 119SnNMRlinewidthandlineshapeareindependentofthemag-neticfieldstrength(B0=7.05to11.75T,FigureS5),indicat-ingnoevidenceofshieldinganisotropy.Furthermore, the119SnNMR linewidthdecreasesslightly (ca.0.8kHz)onceMASisapplied(FigureS6),confirmingasmallcontributionfrom heteronuclear dipolar coupling (through space) be-tweenSnandBrnuclei(79/81Br:I=3/2,totalabundance=100%). These findings suggest that a dominant1J(119Sn−79/81Br)scalarcouplingbetween119Snandthesixdirectlybonded79/81BrnucleiinSnBr6octahedraisrespon-sibleforthe119SnNMRlinewidth.35,36,38,67The119Snspin-lat-ticerelaxationtime(T1)decreasesfromCs2SnCl6(T1=146s)toCs2SnBr6(T1=5.4s)toCs2SnI6(T1=0.170s),adiffer-enceofthreeordersofmagnitude(Figure2candTable1),whichiscomparabletorecentobservationonASnX3perov-skites.16,17Thetrendin119SnT1valueswheretherelaxationdecreasesdramaticallywithincreasinghalideatomicnum-ber(Figure2c)followstheinverseofthetrendinthescalarcouplingstrengthwhere1J(119Sn−127I)>1J(119Sn−79/81Br)>1J(119Sn−35/37Cl).Hencethisinteractionisthoughttobethedominantrelaxationmechanismandexpectedtobefastestfor Cs2SnI6, intermediate for Cs2SnBr6 and slowest forCs2SnCl6,aspreviouslyobservedinothertinhalideperov-skitecompounds.16,68,69Chlorine-35NMRspectroscopyinCs2SnCl6.Thestablehal-ogenelementsareNMRactive,butunlike19F(I=1/2),theheaviergroup17elementsarequadrupolar(nuclearspin,I

=3/2(35/37Cl,79/81Br)orI=5/2(127I))withlargequadrupolemoments(TableS1).35ClNMRspectroscopyhasbeenusedtoinvestigatethelocalstructuralenvironmentsinpharma-ceuticals,aminoacids,leadhalideperovskites,organicandorganometalliccompounds,duetoitsoverallNMRsensitiv-ityaswellastheinsightintostructurethatitprovides.70–75Crystallographically,thehalogenatomsinCs2SnX6occupyasingle site with symmetry 4m.m, where each halogen isbondedtoasingleSnatom(Figure1a).Hence, this lowersitesymmetryoftheXsiteinCs2SnX6isexpectedtoimpartasizableelectricfieldgradient(EFG)thatwouldcausesub-stantialspectralbroadening.Figure3showsthefrequency-steppedwide-band,uniform,andsmoothtruncationCarr-Purcell-Meiboom-Gill (WURST−CPMG)76,77 35Cl NMR spec-trum of Cs2SnCl6. The ultrawideline NMR spectrum thatspansca.2.7MHzat11.75Twasobtainedin12stepsusingthe variable offset cumulative spectra (VOCS) technique(FigureS7).Fittingthecentraltransitionrevealsthatδiso=560±20ppm,CQ=32.0±0.3MHzandthatthequadrupolarasymmetryparameter,η=0.00.Thespectral lineshape isconsistent with the presence of a terminal Cl atom inCs2SnCl6,astheterminalClatomsareinahigheraxialsym-metricenvironment,whichresultsinalargeCQandalowη.Moreover,thelarge35ClCQvalueconfirmsthechlorideionsarecoordinatedtoaSn(IV)ioninalocalSnCl6octahedralenvironment.75Incontrast,ClatomsattachedtoSn(II)ionsshowlowerCQvaluesof<30MHz.75Wenotethatbulkandnanocrystalline formsofCsPbCl3perovskiteexhibit lower35ClCQvaluesofca.15.5MHz(η=0).73

Figure3.Experimental(lowertrace,blue)andsimulated(up-pertrace,red)35ClWURST-CPMGsolid-stateNMRspectrumofCs2SnCl6parent(νr=0kHz,B0=11.75T,sumof12frequencysteps,250kHz/step).SnI6OctahedralDynamicsinCs2SnI6.AnharmoniclatticedynamicsofBX6octahedrainhalideperovskitesplayavitalroleinopticalandelectricalpropertiesofthesematerials.78–80 The isolatedBX6 octahedral units inA2BX6 vacancy-or-dereddoubleperovskitesextend thedegreeofdynamicalfreedomcompared to that for theirsistermaterials,ABX3

perovskites (Figure 4a). Recently, X-ray pair distributionfunctionanalysisonCs2SnI6hasrevealedasymmetryintheinteroctahedralI−Idistance,whichcanbecorrelatedwithrotational disorder for the isolated SnI6 octahedral units,butthetechniquedoesnotprovideinformationabouttheactivationenergyassociatedwith thisrotation.81,82There-fore,variable-temperature119SnNMRmeasurementswerecarried out on a solvent-synthesized Cs2SnI6 sample toprobespecificallytheanharmonicityoftheisolatedoctahe-dralSnI6unitstodeterminetheassociatedactivationener-giesneededforthisuniqueproperty.Asthesampletemperatureincreasesfrom208to380K,the119SnNMRresonanceshiftslinearlytowardslowerfrequen-cies from−4522to−4547ppm(TableS4,Figures4bandS8),attributedtoachangeintheinteratomicdistanceasthetemperature changes. The linewidth of 119Sn NMR reso-nanceremainsnearlyunaltered(fwhm=18±1kHz)acrossthe temperature ranges studied, indicating that the 119SnNMRlinewidthinCs2SnI6isprimarilydrivenbyindirect(J)anddirect(dipolar)spin-spincouplingbetween119Snand127Inuclei.Mostinterestingly,the119SnNMRspin-latticere-laxationtime(T1)changesdrasticallyfrom4.6s(208K)to0.12s(380K);nearlya40-foldchange intheT1overthe170 K temperature range. A plot of ln[T1/s] against[1000/T(K)]islinearallowingadeterminationofanactiva-tionenergyof12.45±0.87kJ/mol(0.129±0.009eV),inac-cordancewiththeArrheniusrelationship(seeTableS4,Fig-ure 4b, Supplementary Note 1), suggesting a low energybarrierfortheSnI6octahedrallatticedynamics.Acompara-bletemperaturedependentchangein119SnT1hasbeenre-portedfortheionicconductors,CH3NH3SnBr3andCsSnBr3perovskites, where much higher activation energies(0.3−0.4eV)wereobservedforrapidhalogenmigration.16,17

Figure 4. (a) Schematic diagram of SnI6 anharmonicity inCs2SnI6structure.(b)Variabletemperature119SnT1data(νr=

0kHz,B0=11.75T)asafunctionofinverseoftemperatureforsolventsynthesizedCs2SnI6.Mechanochemically Synthesized Cs2SnClxBr6−x andCs2SnBrxI6−x Mixed-Halides. Halide perovskites are pre-dominatelypreparedbysolvent-assisted,hightemperatureorvapordepositiontechniques.1,83,84Mechanochemicalsyn-thesisisanentrenched“greener”syntheticroutetoprepareabroadrangeofadvancedfunctionalmaterials,85–87andhasrecentlybeenappliedsuccessfullyinlarge-scaleproductionof perovskite photovoltaic materials.36,38,88–92 Moreover,certain compositions of APbX3 mixed-halide perovskiteswereachievedonlybymechanochemicalsynthesis,butnotviasolvent-assistedroutes.37,38,93Bothmanualhand-grindingusingmortarandpestleandau-tomatedelectricball-millinghavebeenusedassolvent-freemechanochemical synthesis routes topreparebulkhalideperovskites.16,36–38,89,90Recently,wehaveshownthat,usingahand-grindingapproach,onecanobtainphasepureAPbX3perovskiteswithin hours.36–38However, replacing lead(II)with tin(IV), we find that hand-grinding preparation ofCs2SnI6requiresasubstantiallylongermixingtime(6h).Incomparison, electric ball-milling is more efficient (1.5 h)over the labor-intensive manual route (Figure S9). ThephasepureCs2SnClxBr6−x(x=3)andCs2SnBrxI6−x(x=0,1.5,3,4.5,6)sampleswerepreparedbymechanochemicalball-millingunderambientconditions(detailedsynthesisisdis-cussedinSI;seeExperimental,TableS5andFiguresS10-S11).Figure 5a shows photographs of the ball-milledCs2SnClxBr6−xandCs2SnBrxI6−xsamplesandtheirassociatedcolors consistentwith the optical bandgaps. The samplesaremicron-sizedparticleshavinguniformdistributionsofthe constituent elements (Cs, Sn, Cl, Br, I), as determinedfrom elemental mapping images collected using FESEM(Figures5bandS12).Theelementalcompositionsofmixed-halideanalogues,asdeterminedbyEDSanalysis,agreewellwiththenominalcompositions(TableS2andFigureS13).ThepowderXRDpatterns collected at room temperatureareconsistentwiththecubicstructure(spacegroupFm3"m)forCs2SnClxBr6−xandCs2SnBrxI6−x(Figures5candS14).ThepowderXRDpeaksareshiftedtowardsahigherdiffractionangle (Figure 5c), and the refined unit cell parameter in-creasesassmallerhalideionsarereplacedwithlargervari-ants (Table S6). The unit cell parameter in Cs2SnBrxI6−xagainstBrcontentshowsnosignificantdeviationfromline-arity,inagreementwithVegard’slaw(Figure5e).Thisfind-ing suggests that Br and I atoms are randomlymixed inCs2SnBrxI6−x; consistentwithourpreviousobservations inlead-basedmixed-halideperovskites.37,38The optical absorption spectra show that the absorptionedge shifts to lower energy with greater Br content inCs2SnClxBr6−xand I content inCs2SnBrxI6−x (Figure5d).AsextractedfromtheTaucplotofthedirectbandgap,theop-ticalbandgapvaluesspanabout3eVforthesesamples(Ta-bleS6andFigureS15),whichagreeswellwiththepreviousobservations.64,94 The bandgap in the Cs2SnBrxI6−x mixed-halide series decreases nonlinearly from 3.30 eV forCs2SnBr6to1.54eVforCs2SnI6(Figure5e).Thisbehavioris

different fromthe linearbandgaptrendobserved in lead-containingmixed-halide perovskites,36–38 but is similar totheobservedanomalousnonlineartrendinthemixedtin-leadperovskites.95Amorecompletelocalstructuralmodelforthemixed-hal-idesamplescanbeobtainedbysolid-state133Csand119SnNMRspectroscopytointerrogatetheA-andB-sites,respec-tively. Figure 6a shows the 133Cs MAS NMR spectra forCs2SnClxBr6−xandCs2SnBrxI6−x.Cs2SnCl3Br3exhibitsaninho-mogeneouslybroadened133CsNMRresonance(fwhm=1.1kHz) due to the spectral overlap of multiple localCsClmBr12−m (m=0−12)halideenvironmentsaccording totherandomdistribution(TableS7andFigureS16a).36The133CsSSBmanifoldisrelatedtothelocalCsX12cuboctahe-dralsymmetryofperovskites,49anincreaseinthe133CsSSBsinCs2SnCl3Br3 indicatesaperturbationwithin theelectricfieldgradientduetotheloweringinthelocalcuboctahedralsymmetryuponClandBrmixing,andhenceanincreaseinthe quadrupole coupling constant (Figure S17). For theCs2SnBrxI6−x series, the 133CsNMRresonancesspanacross160ppmbetween115to−45ppm.Theresonancesareei-therspectrallyunresolvedforBr-richorresolvedforI-richsamples (Figure 6a). The Br-rich sample, Cs2SnBr4.5I1.5,showsaninhomogeneouslybroadened133CsNMRpeakduetospectraloverlappingofmultiplelocalCsBrmI12−menviron-mentsaccordingtotherandompopulationdistribution(Ta-bleS7andFigureS16b), thereby limitingspectral resolu-tion.Cs2SnBr3I3whichcontainsequalproportionsofBrandIatoms,exhibitsapartiallyspectrallyresolved133CsNMRresonance. Remarkably, the I-rich sample, Cs2SnBr1.5I4.5shows spectrally resolved 133Cs NMR resonances, whichcorrespondtosixdistinguishablyresolvedlocalCsBrmI12−m(m=0,1,2,3,4,5)environments(Figure6b).Thelowestchemicalshiftat−38.4ppm(fwhm=500Hz)isassignedtothe CsI12 environment, which is comparable to the localchemicalenvironmentat−41.5ppm(fwhm=110Hz) forCs2SnI6 parent (Figure6a).Theobservedbroadening andshiftoftheresonancetohigherfrequencyisassociatedwiththe lower site symmetry than cubic as the next-nearestneighborsinthemedium-rangestructure(>5Å)consistsofamixtureofBrandIatoms.49Theremainingpeakswithinthe sets of 133Cs resonances inCs2SnBr1.5I4.5 shift towardshigherfrequencyasIissubstitutedbyBr(Figure6candTa-bleS8),followingastrictlineardependence[Eq(1)]withminCsBrmI12−m. δ(133Cs)/ppm=17.0∙m–36.78 (1)Furthermore,theobservationofincreasing133CslinewidthforCsBrmI12−mpeakswith increasedBrcontent(TableS8)suggeststhespectraloverlapofmultiplepositionalisomersforuniqueCsBrmI12−menvironmentsthatconsistsofamix-tureofBrandIatomsin12positions.The133Csspin-latticerelaxationtimesdecreaselinearlyfrom40to14swithin-crease innumberofBrattachedinCsBrmI12−m(Figure6c).Acquiringthe133CsNMRspectrawithaquantitativerecycledelayforCs2SnBr1.5I4.5(FigureS18),thefitted133Cspeakar-easforeachCsBrmI12−menvironmentcloselyfollowabino-mialpopulationdistribution(Figure6dandTableS8).Thisobservation is consistentwithrandomBrand Imixingatthe local structure, forming an atomic-level mixed-halide

solid-solutionwithoutevidenceofhalogen-richphaseseg-regationordomains.The119SnNMRspectrafortheCs2SnClxBr6−xandCs2SnBrxI6−xseriesarefascinatingand,exhibitbroad(100sofkHz)andpartial spectrallyresolvedmultiple 119Snresonances(Fig-ure 7). To achievemaximum 119Sn NMR sensitivity, opti-mized 119Sn recycle delays and variable offset cumulativespectra(ifrequired)wereusedduetotheirdrasticdiffer-encesin119SnT1andlargechemicalshiftspans,respectively

(TableS9d).Thisobservationcontrastswiththerecentob-servationsonASnX3mixed-halideperovskites,wheresingle119Snresonancesarereportedduetorapidhalogendynam-ics.16,17However,thebroadeningandmultiresonancespec-tralbehavioragreeswellwith207PbNMRspectraforAPbX3mixed-halideperovskites.36–38

Figure5.(a)Photographsofball-milledsamples:(i)Cs2SnCl3Br3,(ii)Cs2SnBr6,(iii)Cs2SnBr1.5I4.5,(iv)Cs2SnBr3I3,(v)Cs2SnBr1.5I4.5and(vi)Cs2SnI6.(b)ScanningelectronmicrographandelementalmappingforCs2SnBr3I3.(c)PowderXRDpatterns,(d)normalizedopticalabsorptionspectraofball-milledCs2SnClxBr6−xandCs2SnBrxI6−xsamples(exceptCs2SnCl6,solventsynthesized).(e)Plotsofunitcellparametersandbandgapsofball-milledCs2SnBrxI6−xseries.

Figure6.(a)Solid-state133Cs(νr=13kHz,B0=11.75T)NMRspectraofball-milledCs2SnClxBr6−xandCs2SnBrxI6−xsamples(exceptCs2SnCl6,solventsynthesized).(b)Experimentalandfitted133CsNMRspectra,(c)133Cschemicalshiftand133Csfwhmvaluesand(d)populationdistributionasafunctionofBratomsinCsBrmI12−mforCs2SnBr1.5I4.5.

Figure7.Solid-state119Sn(non-spinning,B0=7.05T)NMRspectraof(a)ball-milledCs2SnClxBr6−xandCs2SnBrxI6−x(exceptCs2SnCl6,solventsynthesized),(b)Cs2SnCl3Br3and(c)Cs2SnBr4.5I1.5.ThelocalSnClnBr6−nandSnBrnI6−nenvironmentsareassignedin(b)and(c).Tounderstandtheobserved119SnNMRspectraformixed-halideanalogues,the119Snmagneticshieldingparametersofisolatedoctahedralclusters,i.e.,SnClnBr6−n(n=0−6)forCs2SnClxBr6-xandSnBrnI6−n forCs2SnBrxI6-xwerecomputedusingDFTandthecomputationaldetailsareprovidedintheSI(seeExperimental,TablesS10-S11).Thecomputed119Snisotropic chemical shifts are linearly shifted to lower fre-quencyasClissubstitutedbyBrinSnClnBr6−n(Figure8a)andBrissubstitutedbyIinSnBrnI6−n(FigureS19).Thecom-putedmagneticshieldinganisotropyparameters(spanandskew) exhibit awide range of values that exclusively de-pendonnvaluesandtheirpositionalisomers(cis/transorfac/mer)inSnClnBr6−nandSnBrnI6−n;thisisresponsibleforbroaderNMRlinewidths(Figures7,8andS20).Forexam-ple,thecomputed119Snspanis1058ppmfortrans-SnCl2Br4in Cs2SnClxBr6−x and up to 4702 ppm for trans-SnI4Br2 inCs2SnBrxI6−x.Theexperimentallyobserved119SnNMRspec-trainCs2SnClxBr6−xandCs2SnBrxI6−xareacombinationofin-dividual resonances of all possible SnClnBr6−n or SnBrnI6−nspeciesaccordingtotheirpopulationsfollowingarandomdistribution(TableS12andFigureS21).Alongwithmag-netic shieldinganisotropy, theexperimentalNMRspectragetfurtherbroadenedbybothindirectanddirectspin-spincoupling between 119Sn and six quadrupolar halogens(35/37Cl,79/81Brand127I)inSnClnBr6−norSnBrnI6−n,videsupra.Duetothecomplexnatureoftheseparameters,inabilitytoacquire ultrafastMAS and less spectral resolution among

tenpossibleuniquehalidearrangementsincludingtheiso-mersinSnClnBr6−norSnBrnI6−n,werefrainfromquantitivelydeconvoluting the experimental 119Sn NMR spectra forCs2SnClxBr6−xandCs2SnBrxI6−xbutthesearediscussedqual-itativelybelow.The multiple 119Sn peaks in mixed-halide samples corre-spondtoseveraldistinctlocalSnoctahedralenvironments.GuidedbytheDFTcomputations,thefivespectrallydistin-guishable 119SnNMRresonances inCs2SnCl3Br3 are tenta-tivelyassignedasthespectralregionsforSnCl6(−707ppm),SnCl5Br1/SnCl4Br2 (−966 ppm), SnCl3Br3 (−1291 ppm),SnCl2Br4/SnCl1Br5 (−1610 ppm) and SnBr6 (−1962 ppm)(Figure 7b). The 119Sn NMR spectra for Cs2SnBrxI6−x getmuch broader and unresolved due to the stronger1J(119Sn−79/81Br) and 1J(119Sn−127I) in SnBrnI6−n along withtheir higher magnetic shielding anisotropy (1000s ppm)predictedbyDFT(TableS10).Nevertheless,fourspectrallyresolved119SnNMRresonancescanbetentativelyassignedin Br-rich Cs2SnBr4.5I1.5 as the spectral regions for SnBr6(−1962ppm),SnBr5I1(−2434ppm),SnBr4I2(−2842ppm)andSnBr3I3(−3378ppm)environments(Figure7c).Influence of Synthesis Routes on Local Structure ofCs2SnI6.A featureofphotovoltaicperovskites is that theycanbesynthesizedbyvariousavenues,withorwithoutsol-vents;however,thelocalchemicalstructureresultingfromtheseapproaches ispoorlyunderstood.Thus, toelucidate

thestructure,solid-state133Csand119SnNMRspectroscopywasperformedforthreeCs2SnI6samplespreparedbythreedifferentpreparationtechniques,namely,solvent-freeball-milling, hand-grinding, and solvent-assisted synthesisroutes.

Figure8.DFTcomputedresultsforSnClnBr6−n:(a)optimizedgeometries, (b)119Snisotropicchemicalshiftasa functionofnumberofCland(b)simulated119SnNMRspectra.The133CsNMRspectraofthesethreeCs2SnI6samplesshowresonancesinthesamespectralpositionswithnearlyiden-ticallinewidths(Figure9a).However,theslightasymmetryoftheunderlying133Csresonancefortheball-milledsamplecorrespondstohigherlocalstructuraldisorderandsmalleraverage crystallite sizes as compared to solvent-synthe-sizedandhand-groundsamples.The119SnNMRspectraforallthreeCs2SnI6samplesshowsymmetricresonanceswithdifferent chemical shifts and linewidths (Figure 9b). Thesolvent-synthesized and hand-ground samples exhibitsharper resonances at −4535 ppm (fwhm= 17 kHz) and−4527 ppm (fwhm = 17 kHz), respectively, whereas, theball-milledsampleshowsabroadresonanceat−4505ppm(fwhm = 20 kHz). The sharper 119Sn resonance indicatesthat the solvent-synthesized sample has a well-orderedstructurewithminimumdefects,whereas,thebroader119Snresonanceindicatesthepresenceofthehigherdefectcon-centration in the ball-milled sample. The ball-milling is ahigh energy grinding technique that causes grinding-in-duceddefectsascomparedtothelowenergymanualhand-grinding route. The 119Sn spin-lattice relaxation dependssignificantlyonthesynthesisrouteswithT1=0.01sfortheball-milledsample,whichisanorderofmagnitudelessthanthatofthesolventsynthesizedcounterpart.Thefasterre-laxationprocessfortheball-milledsamplesupportshigher

localstructuraldisorderordefectswhichisconsistentwiththeconclusionbasedon119SnNMRlinewidths.ThehigherstructuraldisorderisalsoreflectedinbroaderpeaksinpowderXRDpatternsfortheball-milledsampleascomparedtothewellstructurallyorderedsolvent-synthe-sizedsample(FigureS22).Thedifferentdegreesofcrystal-linityinCs2SnI6samplesalsoaffectstheopticalabsorptionproperties, with bandgaps of 1.56 eV for the ball-milledsampleand1.42eVforthesolvent-synthesizedsample.

Figure9.Solid-state(a)133Cs(νr=5kHz,B0=11.75T)and(b)119Sn(non-spinning,B0=7.05T)NMRspectraofCs2SnI6sam-plespreparedby solvent-synthesis (SS), hand-grinding (HG),andball-milling(BM)routes.

CONCLUSIONS Themacrotomicroscopicstructuresandbulkphotophysi-calpropertiesforCs2SnX6(X=Cl,Br,I)parentperovskitespreparedbythesolvent-assistedsynthesisroutewereeval-uatedusingpowderXRDandmultinuclearmagneticreso-nancespectroscopy.Tin-119NMRspectroscopyrevealedamassivechemicalshiftrangespanningca.4000ppmthat,according to DFT computations, is dominated by a largespin-orbitcomponent.Likewise,the119Snspin-latticerelax-ationtimeschangebythreeordersofmagnitudeforthesematerials as the halide anion increases in size. The 35ClquadrupolecouplingconstantforCs2SnCl6isnearlydoublethatforthehybridandnon-hybridlead(II)chlorideperov-skites,APbCl3,consistentwithClboundtoSnina4+oxida-tion state.The rapidoctahedraldynamicsof SnI6 units inCs2SnI6areremarkable,displayingalowactivationenergy(12.45± 0.87 kJ/mol) for the polyhedralmotion in thesestable“static”solids.ThecompletesolidsolutionsCs2SnClxBr6−xandCs2SnBrxI6−x(0 ≤ x ≤ 6) mixed-halides that exhibit attractive optical

bandgapsspanningapproximately3eV,weresuccessfullyprepared by a solvent-free ball-milling synthesis route atroomtemperature.Accordingtosolid-state133Csand119SnNMRspectroscopy,themixed-halideanaloguesundergoacompleterandomdistributionofCl/BrorBr/Ihalogensatthe atomic level, with no evidence of halogen-rich phasesegregation or domains under dark conditions. Further-more,thebroad119SnNMRresonancesobservedinmixed-halidesamples, supportedbymagnetic shieldingparame-tersobtainedbyDFT,assisted inassigning theuniqueSnchemicalenvironments,supportingthecomplexnatureonemust contend with due to considerable shielding anisot-ropy.ThebulkopticalbandgappropertiesfortheCs2SnI6samplesprepared by solvent-free and solvent-assisted synthesisroutesaresensitivetolocalstructuralchangesasconfirmedbyNMR.Forexample,materialspreparedbyhighenergyball-millingtendtoshowalowerdegreeofcrystallinity(i.e.,higherlocalstructuraldisorder),resultinginfaster119Snre-laxationandaslightincreaseinthebandgap(0.14eV)com-pared to the solvent-synthesized counterpart. Overall,solid-stateNMRspectroscopy,alongsidePXRDandDFTbe-stowsunprecedentedabilitytounravelthecomplexchemi-calstructureanddynamicspresentwithinvacancy-ordereddouble perovskite materials, whereby random halide ar-rangementsanddefectsgoverntheopticalbandgaps.

ASSOCIATED CONTENT SupportingInformation.Detaileddiscussionofsynthesis,experimentaltechniques,DFTcomputations,TablesS1-S12andFiguresS1-S22areavailableintheSupportingInformation(PDF).

AUTHOR INFORMATION

Corresponding Author *e-mail:vladimir.michaelis@ualberta.ca

Author Contributions Allauthorshavegivenapprovaltothefinalversionofthemanuscript.Notes Theauthorsdeclarenocompetingfinancialinterest.

ACKNOWLEDGMENT We acknowledge the Natural Sciences and Engineering Re-searchCouncil(NSERC)ofCanada,CanadaFoundationforIn-novation, Government of Alberta, Future Energy Systems(CFREF),CREATE(ATUMS)and theUniversityofAlberta forresearchsupport.AKacknowledgesanAlbertaInnovatesGrad-uateScholarship.VAMGissupportedbyaConsejoNacionaldeCiencia y Tecnologia (CONACyT), scholarship No. 291163. Apart of the research was carried out on computational re-sources provided by Compute Canada/ Calcul Canada(www.computecanada.ca).

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TOCgraphic

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S1

Supporting Information

Uncovering Halide Mixing and Octahedral Dynamics in Vacancy-Ordered Double Perovskites by Multinuclear Magnetic Resonance Spectroscopy Abhoy Karmakar, Srija Mukhopadhyay, Pierre G.B. Gachod, Victor Arturo Mora-Gomez, Guy M. Bernard, Alex Brown and Vladimir K. Michaelis*

Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada

*corresponding author: vladimir.michaelis@ualberta.ca

Table of contents Contents

Page(s)

Experimental S3-S6 Table S1. Naturally occurring NMR-active isotopes in Cs2SnX6. S6 Table S2. Elemental composition analysis by EDS. S7 Table S3. Paramagnetic, diamagnetic and spin-orbit components of the total isotropic shielding tensor of tin(IV) atom of Cs2SnX6 (X = Cl, Br, I) at the ZORA/PBE0/TZ2P including spin-orbit level of theory.

S7

Table S4. Variable temperature 119Sn NMR parameters for solvent synthesized Cs2SnI6 sample at 11.75 T under non-spinning sample condition.

S7

Supplementary note 1 S8 Table S5. Starting materials and their weights used for ball-mill preparation of the Cs2SnClxBr6-x and Cs2SnBrxI6-x samples

S8

Table S6. Unit cell parameters and direct bandgap values for ball-milled samples. S8 Supplementary note 2 S8 Table S7. Calculated population distributions (binomial) for CsClmBr12−m and CsBrmI12−m environments in Cs2SnClxBr6−x and Cs2SnBrxI6−x (0 ≤ x ≤ 6).

S9

Table S8. Lorentzian profile fitted 133Cs MAS NMR spectra for CsBrmI12−m environments in Cs2SnBr1.5I4.5.

S10

Table S9. Solid-state 133Cs and 119Sn NMR experimental parameters. S10-S12 Table S10. DFT computed NMR shielding parameters for structurally optimized SnClnBr6−n and SnBrnI6−n (n = 0−6) octahedral species determined using PBE0/TZ2P, incorporating relativistic effects with ZORA and spin-orbit coupling.

S12

Table S11. Optimized Cartesian coordinates of SnClnBr6−n and SnBrnI6−n (n = 0−6) octahedral species described in Table S10.

S12-S14

Table S12. Calculated population distribution (binomial) for SnClnBr6−n and SnBrnI6−n environments in Cs2SnClxBr6−x and Cs2SnBrxI6−x (0 ≤ x ≤ 6).

S15

Figure S1. Elemental analysis via energy dispersive X-ray spectroscopy for the solvent synthesized Cs2SnX6 (X = Cl, Br, I) parents as indicated.

S16

Figure S2. FESEM images for the solvent synthesized Cs2SnX6 (X = Cl, Br, I) parents. S17 Figure S3. Tauc plots showing direct bandgaps of the solvent synthesized Cs2SnX6 (X = Cl, Br, I) parents.

S17

Figure S4. Solid-state 133Cs MAS NMR spectra (frequency scale) of the solvent synthesized Cs2SnX6 (X = Cl, Br, I) parents at 11.75 T and νr = 5 kHz.

S17

Figure S5. Solid-state 119Sn NMR spectra (frequency scale) of solvent synthesized Cs2SnBr6 at 7.05 and 11.75 T under MAS (νr = 10 kHz) conditions.

S18

Figure S6. Solid-state 119Sn NMR spectra (frequency scale) of solvent synthesized Cs2SnBr6 at 7.05 T under non-spinning and MAS (νr = 10 kHz) conditions.

S18

S2

Figure S7. Solid-state 35Cl NMR subspectra with different frequency offsets (250 kHz step size) acquired at 11.75 T using a frequency-stepped WURST-CPMG pulse sequence.

S19

Figure S8. Temperature dependent 119Sn NMR spectra (νr = 0 kHz, B0 = 11.75 T) for solvent synthesized Cs2SnI6 sample.

S20

Figure S9. Powder XRD patterns of ball-mill and hand-grind synthesized Cs2SnI6 materials at different grinding times as indicated.

S20

Figure S10. Powder XRD patterns for ball mill synthesized Cs2SnBr6 materials with different CsBr and SnBr4 molar ratios.

S21

Figure S11. Powder XRD patterns of (a) Cs2SnCl3Br3 and (b) Cs2SnBr3I3 materials after 1.5 and 3.0 h of ball milling.

S21

Figure S12. FESEM image and the corresponding EDS elemental mapping for Cs, Sn, Cl, Br and I for the ball-milled Cs2SnCl3Br3, Cs2SnBr6, Cs2SnBr4.5I1.5 and Cs2SnBr1.5I4.5 samples as indicated.

S22

Figure S13. Elemental analysis via energy dispersive X-ray spectroscopy for the ball-milled Cs2SnClxBr6−x (x = 3) and Cs2SnBrxI6−x (x = 0, 1.5, 3, 4.5, 6) samples as indicated.

S22-S24

Figure S14. The experimental and fitted powder XRD diagrams of ball-milled Cs2SnClxBr6−x (x = 3) and Cs2SnBrxI6−x (x = 0, 1.5, 3, 4.5, 6) samples. All the diffraction data are fitted assuming a cubic (Fm𝟑"m) space group symmetry.

S25

Figure S15. Tauc plots showing direct bandgaps of ball-milled Cs2SnClxBr6−x (x = 3) and Cs2SnBrxI6−x (x = 0, 1.5, 3, 4.5, 6) samples.

S26

Figure S16. Calculated population distributions (binomial) of local cuboctahedral environments: (a) CsClmBr12−m in Cs2SnClxBr6−x (x = 0, 3, 6) and (b) CsBrmI12−m in Cs2SnBrxI6−x (x = 0, 1.5, 3, 4.5, 6).

S27

Figure S17. Solid-state 133Cs MAS NMR spectra of Cs2SnCl6, Cs2SnCl3Br3 and Cs2SnBr6 at 11.75 T with a MAS frequency of 13 kHz.

S28

Figure S18. Normalized 133Cs MAS NMR spectra (B0 = 11.75 T, νrot = 13 kHz) exhibiting spectral overlap of multiple resonances with recycle delays of 10 and 850 s for Cs2SnBr1.5I4.5. The 133Cs NMR spectra were acquired using a short tip angle of 1.38 µs (5.5 µs π/2) Bloch pulse sequence.

S28

Figure S19. DFT computed results for SnBrnI6−n (n = 0−6) environments: (a) optimized geometries and (b) 119Sn isotropic chemical shift as a function of the number of attached Br atoms in SnBrnI6−n.

S29

Figure S20. Simulated 119Sn NMR spectra for SnBrnI6−n (n = 0−6) environments. The anisotropic parameters were obtained from DFT computation.

S29

Figure S21. Calculated population distributions (binomial) for local octahedral environments: (a) SnClnBr6−n in Cs2SnClxBr6−x (x = 0, 3, 6) and (b) SnBrnI6−n in Cs2SnBrxI6−x (x = 0, 1.5, 3, 4.5, 6).

S30

Figure S22. Powder XRD patterns of Cs2SnI6 samples prepared by ball-milling (BM) and solvent-synthesis (SS) routes.

S31

References S32

S3

EXPERIMENTAL

All starting precursors and solvents were purchased from commercial sources and used directly

without further purification: CsCl (Terochem Laboratories Ltd., 99.7%), CsBr (Sigma, 99%), CsI (Sigma,

99.9%), anhydrous SnCl4 (Sigma, 98%), SnBr4 (Alfa Aesar, 99%), SnI4 (Alfa Aesar, 95%), HCl (Caledon

Laboratory Chemicals), HBr (Anachemia, 48%), and acetonitrile (Sigma, 99.5%).

A. Synthesis:

I. Solvent synthesis of Cs2SnX6 (X = Cl, Br and I):

Cs2SnCl6 and Cs2SnBr6: The starting materials, SnX4 (1 mmol) and CsX (2 mmol) (X = Cl, Br)

were weighed, taken in a clean vial, and 5 mL concentrated HX (aq) was added to the mixture. The solution

mixture was heated using a hot plate set to 120 °C and stirred at 300 rpm continuously for one hour using

a magnetic stir bar. The solution was cooled to room temperature over a period of two hours. The

precipitates were filtered by vacuum filtration using a Buchner funnel, followed by washing with 95%

ethanol, and allowed to dry overnight. The samples were stored in clean sealed vials under ambient

conditions for further use.

Cs2SnI6: Cs2SnI6 was synthesized following previous report with some modification.1 SnI4 (0.25

mmol) and CsI (0.5 mmol) starting precursors were weighed and taken in the molar ratio of 1:2 in a clean

vial and 2 mL of acetonitrile solvent was added to the mixture. The solution mixture was stirred for three

hours until a homogeneous mixture was obtained using a magnetic stir bar. Then the solvent was boiled off

by heating the vial over the hotplate set to 80 oC. The remaining black powder was stored in a sealed vial

under ambient conditions until needed.

II. Mechanochemical synthesis of Cs2SnXxX’6-x (X, X’ = Cl/Br, Br, Br/I, I):

Ball-mill synthesis: The starting materials to prepare Cs2SnClxBr6−x (x = 3) and Cs2SnBrxI6−x (x =

0, 1.5, 3, 4.5, 6) samples were first ground using an agate mortar and pestle for 5−10 minutes. The mixture

was then transferred to a 50-mL zirconia grinding vessel, containing ca. 50 g of zirconia balls (3−8 mm

diameter) and sealed under ambient conditions. The sample was ground in an electric planetary ball-mill

instrument (Changsha Deco-PBM-V-0.4L) at a rotation frequency of 550 rpm for 0.5 h. The inner wall of

the vessel was scraped by using a clean spatula and sealed again. This process was repeated to achieve a

total 1.5 or 3 h of grinding time.

Cs2SnBr6 and Cs2SnI6 parents were prepared by ball-milling from solid CsX and SnX4 (X = Br, I)

binary precursor salts; however, Cs2SnCl6 was prepared only by the solvent-assisted synthesis route to avoid

handling of SnCl4 which is a hygroscopic liquid. We note an excess SnBr4 may be required for the ball-

milling preparation to obtain phase pure Cs2SnBr6 (Figure S10), due to the compound’s physical properties

exhibiting both a low melting point (31 oC) and a hygroscopic nature, whereas a phase-pure Cs2SnI6 can

S4

easily be obtained by ball-milling with proper starting precursor compositions (CsI : SnI4 = 2 : 1) due to

SnI4’s considerably higher melting point of 144 oC. Therefore, to avoid the direct use of SnCl4 and SnBr4

as starting precursors in our ambient condition ball-milling preparation, Cs2SnCl3Br3 (x = 3) mixed-halides

were synthesized from Cs2SnCl6 and Cs2SnBr6 parents, while Cs2SnBrxI6−x (x = 1.5, 3.0, 4.5) mixed-halides

were obtained from the appropriate molar ratios of Cs2SnBr6, CsI, and SnI4 (Table S2). On the other hand,

the grinding time is a crucial parameter in the ball-milling process. The phase pure product of Cs2SnBr6 and

Cs2SnI6 parents were obtained in 1.5 h, whereas Cs2SnClxBr6−x and Cs2SnBrxI6−x mixed-halide analogues

required grinding times of 3 h under the same experimental conditions (Figure S11). The quantities of the

starting materials and grinding times are summarized in Table S5.

Hand-grind synthesis of Cs2SnI6: A mixture of 2 mmol of CsI and 1 mmol of SnI4 were hand

ground using an agate mortar and pestle for up to 6 h until phase-pure Cs2SnI6 was obtained. We note that

the targeted phase pure Cs2SnI6 sample was not obtained after up to 4 h of grinding time as confirmed by

powder XRD (Figure S9).

B. Powder X-ray Diffraction (XRD): The powder XRD patterns were collected on a Rigaku Ultima IV

powder diffractometer equipped with a Co Kα (Kα1 = 1.78900 Å and Kα2 = 1.79283 Å) radiation source at

38 kV and 38 mA, and a D/Tex Ultra detector with Fe filter to eliminate the Kβ (= 1.62083 Å) radiation. All

samples were loaded on zero background plates and the data were collected in continuous scan mode

between 5° to 90° in 2q angle with a step size of 0.0200°. All the data presented in the main text were

converted to a Cu Kα (Kα1 = 1.54056 Å and Kα2 = 1.54439 Å). Profile fitting of the powder XRD data was

performed using the FullProf suite software and the unit cell parameters were refined.

Some complementary powder XRD patterns (Figures S9) were collected on an Inel multi-purpose

diffractometer equipped with a Cu Kα (= 1.540596 Å) radiation source at 40 kV and 50 mA, and a CPS 120

detector.

C. Energy-dispersive X-ray spectroscopy (EDS) and Field Emission Scanning Electron Microscopy

(FESEM): The samples were examined on a Zeiss Sigma 300 VP field emission scanning electron

microscope instrument equipped with dual silicon drift detectors for EDS to determine chemical

compositions.

D. Diffuse Reflectance Spectroscopy: The diffuse reflectance spectra for all the samples were collected

using a Cary 5000 UV−vis−NIR spectrophotometer between the wavelength range of 2000-200 nm and

calibrated using a Spectralon (>99%) reflectance standard. The diffuse reflectance data were converted to

pseudo-absorbance spectra using the Kubelka-Munk transformation2 through: α ~ (1-R)2/(2R), where, R =

absolute reflectance and α = pseudo-absorbance. Direct bandgaps were determined from the Tauc plot upon

extrapolation of the intercept value in a (αh𝜈)2 vs E(eV) plot.

S5

E. Solid-State Nuclear Magnetic Resonance (NMR) Spectroscopy:

I. Cesium-133 NMR spectroscopy: Solid-state 133Cs NMR spectroscopy was performed at 295 K on a

Bruker Avance NEO 500 spectrometer (B0 = 11.75 T) under magic-angle spinning (MAS) conditions using

a 4 mm H/X MAS Bruker probe (ω0/2π(133Cs) = 65.6 MHz). The samples were packed into 4 mm (outer

diameter) zirconia rotors with Kel-F® caps. All 133Cs NMR data were acquired using a 1.38 µs π/2 (γB1/2π

= 45.5 kHz) Bloch decay pulse sequence with an optimized recycle delay. Detailed experimental parameters

for 133Cs NMR measurements are shown in Tables S9a, S9c and S9e. All the 133Cs NMR spectra were

referenced with a 0.1 M CsNO3 (aq.) solution at (133Cs) = 0.00 ppm.

II. Tin-119 NMR spectroscopy: Solid-state 119Sn NMR measurements were performed at B0 = 7.05 and

11.75 T on a Bruker Avance NEO 300 and 500 spectrometers, respectively, under both MAS and non-

spinning sample conditions using a 4 mm H/X MAS Bruker probe. All samples were packed into 4 mm

(outer diameter) zirconia rotors with Kel-F® caps. The 119Sn NMR spectra were acquired using either a 4.0

µs π/2 (γB1/2π = 62.5 kHz) or a 1.7 µs π/2 (γB1/2π = 147.1 kHz) Hahn-echo pulse sequence ((π/2)x − τ1 −

(π)y − τ2 − ACQ, where τ represents the inter-pulse and refocusing delay) with an optimized recycle delay.

Detailed experimental parameters for 119Sn NMR measurements are presented in Tables S9b, S9d and S9f.

Variable temperature (200 < T (K) < 400) 119Sn NMR measurements were performed on a Bruker

Avance Neo 500 (B0 = 11.75 T) using a 4 mm H/X MAS double resonance probe under non-spinning

sample conditions. The sample was packed into a 4 mm (outer diameter) zirconia rotors with a vespel cap.

Temperatures were controlled using a Bruker VT unit whereby liquid nitrogen was used for cooling. The

sample temperature was calibrated using the 207Pb chemical shifts of CH3NH3PbCl3 powder.3

The 119Sn spin-lattice relaxation time (T1) was measured using an inversion recovery pulse sequence

(π – τD − π/2 − τ − ACQ, τD is the variable delay). All 119Sn T1 values were calculated by fitting the peak

intensity values using a mono-exponential decay function: It = I∞ + 𝐴𝑒–"/$!, where It and I∞ are the NMR

signal intensities measured at time t and at infinity, respectively, and the pre-exponential constant and spin-

lattice relaxation time are A and T1, respectively.

All 119Sn NMR spectra were referenced by tetracyclohexyl-tin(IV) powder at 𝛿(119Sn) = −97.35

ppm and a secondary reference with respect to Sn(CH3)4 at 𝛿(119Sn) = 0.00 ppm.

III. Chlorine-35 NMR spectroscopy: Solid-state 35Cl NMR measurement for Cs2SnCl6 was performed at

295 K on a Bruker Avance NEO 500 spectrometer (B0 = 11.75 T) under non-spinning sample conditions

using a 4 mm H/X MAS Bruker probe (ω0/2π(35Cl) = 47−50 MHz). A powdered sample was packed into a

4 mm (outer diameter) zirconia rotor fitted with a Kel-F® cap. The 35Cl NMR spectra were acquired using

a frequency-stepped WURST−CPMG pulse sequence4,5 (50 µs pulse width, 512 scans, 50 echoes, 5 kHz

S6

spikelet) and the overall spectra was constructed by taking the skyline of 12 subspectra (Figure S7). 35Cl

NMR spectra were referenced with 0.1 M NaCl (aq) at 𝛿(35Cl) = 0.00 ppm.

F. DFT Computations: The structures and properties of the SnClnBr6−n and SnBrnI6−n (n = 0−6)

environments were obtained using density functional theory (DFT), carried out using ADF.6–8 The

geometries were optimized using the PBE0 exchange-correlation functional9 with the TZ2P basis sets,10

chosen to avoid the convergence issues and the computational cost of larger basis sets. Thus, the gradient

convergence criteria of the geometry optimizations have been refined to 1.0´10−7. Optimizations have been

done at the relativistic scalar ZORA level of theory.11,12 Shielding tensor constants were accomplished using

the PBE0 functional and TZ2P basis set. Shielding tensors for SnClnBr6−n and SnBrnI6−n (n = 0−6) octahedral

clusters have been calculated using the FXC parameter13 (unless otherwise specified) at the relativistic spin-

orbit ZORA level of theory.14 The 119Sn NMR spectra were simulated by using WSolids NMR simulation

package (WSolids1, Version: 1.21.3, 2015, K. Eichele, Universität Tübingen).

Table S1. Naturally occurring NMR-active isotopes in Cs2SnX6.15

Element Isotope(s) Nuclear spin (I)

Natural abundance (%)

Magnetogyric ratio (γ/107 rad s−1 T−1)

Frequency ratio (Ξ/%)

Quadrupolar moment (Q/fm2)

Cs 133Cs 7/2 100 3.5332539 13.116142 −0.343

Sn 115Sn 1/2 0.34 −8.8013 32.718749 N.A. 117Sn 1/2 7.68 −9.58879 35.632259 N.A. 119Sn 1/2 8.59 −10.0317 37.290632 N.A.

Cl 35Cl 3/2 75.78 2.624198 9.797909 −8.165 37Cl 3/2 24.22 2.184368 8.155725 −6.435

Br 79Br 3/2 50.69 6.725616 25.053980 31.3 81Br 3/2 49.31 7.249776 27.006518 26.2

I 127I 5/2 100 5.389573 20.007486 −71.0

S7

Table S2. Elemental composition analysis by EDS.

Samples Synthesis route Atom% Halogen atomic ratio Cs Sn Cl Br I

Cs2SnCl6 Solvent-assisted 21.7 11.0 67.3 - - 100% Cl Cs2SnBr6 20.5 10.2 - 69.3 - 100% Br Cs2SnI6 20.0 12.5 - - 67.5 100% I Cs2SnI6 Hand-grinding 18.5 12.6 - - 68.9 100% I

Cs2SnCl3Br3

ball-milling

21.2 10.9 31.7 36.2 - 47% Cl : 53% Br Cs2SnBr6 21.1 10.5 - 68.3 - 100% Br

Cs2SnBr4.5I1.5 23.8 9.8 - 53.7 12.7 81% Br : 19% I Cs2SnBr3I3 26.2 11.3 - 34.8 27.7 56% Br : 44% I

Cs2SnBr1.5I4.5 24.3 11.2 - 15.7 48.8 24% Br : 76% I Cs2SnI6 20.2 11.8 - - 68.0 100% I

Table S3. Paramagnetic, diamagnetic and spin-orbit components of the total isotropic shielding tensor of tin(IV) atom of Cs2SnX6 (X = Cl, Br, I) at the ZORA/PBE0/TZ2P including spin-orbit level of theory without FXC option. At experimental geometries (at optimized geometries in parentheses)

Shielding tensor Cs2SnCl6 Cs2SnBr6 Cs2SnI6 Paramagnetic −2192.958

(-2197.985) −2286.456 (-2283.346)

−2257.522 (-2292.756)

Diamagnetic 5109.444 (5109.790)

5105.075 (5104.772)

5108.829 (5108.431)

Spin-orbit 890.972 (872.728)

2095.717 (2492.755)

5395.917 (5571.179)

Total 3807.458 (3784.533)

4914.336 (5314.181)

8247.225 (8386.854)

Total (with FXC) 3853.663 (3828.557)

4991.516 (5407.830)

8159.092 (8219.273)

Table S4. Variable temperature 119Sn NMR parameters for solvent synthesized Cs2SnI6 sample at 11.75 T under non-spinning sample condition.

Calibrated temperature

119Sn chemical shift (± 2 ppm)

119Sn fwhm (± 1 kHz)

119Sn T1 (s)

207.5 −4522 17.0 4.63 ± 1.26 212.2 −4524 17.0 2.48 ± 0.41 252.5 −4530 18.5 0.818 ± 0.130 294.1 −4535 18.4 0.577 ± 0.106 328.6 −4542 18.2 0.264 ± 0.098 355.2 −4545 18.7 0.192 ± 0.034 379.9 −4547 18.0 0.117 ± 0.021

S8

Supplementary Note 1: Between the temperature range of 208 to 380 K, the data of ln(T1/s) vary linearly with [1000/T(K)] following an Arrhenius relationship [equation: ln(T1/s) = (1.49769 ± 0.10503)×%&&&

'()) –

(5.92217 ± 0.39057)], as shown in Figure 4b in the main text. The slope of the Arrhenius plot is related to the activation energy as Ea = slope × 1000R, where R = 8.314 J/(mol⸱K). Therefore, Ea = 1.49769 × 1000 × 8.314 J/mol = 12.45 ± 0.87 kJ/mol or 0.129 ± 0.009 eV.

Table S5. Starting materials and their weights used for ball-mill preparation of the Cs2SnClxBr6-x and Cs2SnBrxI6-x samples.

Targeted samples Starting materials and weights Ball-milling time (h)

Cs2SnCl3Br3 Cs2SnCl6 (179 mg) + Cs2SnBr6 (259 mg) 3.0

Cs2SnBr6 CsBr (425 mg) + SnBr4 (505 mg)a 1.5

Cs2SnBr4.5I1.5 Cs2SnBr6 (388 mg) + CsI (78 mg) + SnI4 (94 mg) 3.0

Cs2SnBr3I3 Cs2SnBr6 (259 mg) + CsI (155 mg) + SnI4 (188 mg) 3.0

Cs2SnBr1.5I4.5 Cs2SnBr6 (129 mg) + CsI (234 mg) + SnI4 (281 mg) 3.0

Cs2SnI6 CsI (261 mg) + SnI4 (314 mg) 1.5

a – molar ratio of CsBr/SnBr4 is 2/1.15 in order to obtain a pure phase product as confirmed by powder XRD (see Figure S8).

Table S6. Unit cell parameters and direct bandgap values for ball-milled samples. Samples Cell parameter (a/Å) Bandgap (eV)

Cs2SnCl3Br3 10.6334(3) 3.53 Cs2SnBr6 10.8306(3) 3.30

Cs2SnBr4.5I1.5 11.0835(9) 2.25 Cs2SnBr3I3 11.3218(11) 1.80

Cs2SnBr1.5I4.5 11.5029(9) 1.67 Cs2SnI6 11.6441(4) 1.56

Supplementary Note 2. The binomial population can be calculated as: P(k) = +!-!(+.-)!

pk ∙ (1−p)a−k, where a = number of trials and p = probability of the given trial. For example: (i) CsBrmI12−m in Cs2SnBrxI6−x: a = 12; k = m = 0, 1, 2, 3, …, 10, 11, 12; p = interhalogen mole fraction = 0, 0.25, 0.50, 0.75, 1. (ii) SnBrnI6−n in Cs2SnBrxI6−x: a = 6; k = n = 0, 1, 2, 3, 4, 5, 6; p = interhalogen mole fraction = 0, 0.25, 0.50, 0.75, 1.

S9

Table S7. Calculated population distributions (binomial) for CsClmBr12−m and CsBrmI12−m environments in Cs2SnClxBr6−x and Cs2SnBrxI6−x (0 ≤ x ≤ 6).

m in CsClmBr12−m

CsClmBr12−m Calculated populations (%) Cs2SnCl6 Cs2SnCl3Br3 Cs2SnBr6

0 CsBr12 - 0.02 100 1 CsCl1Br11 - 0.29 - 2 CsCl2Br10 - 1.61 - 3 CsCl3Br9 - 5.37 - 4 CsCl4Br8 - 12.09 - 5 CsCl5Br7 - 19.34 - 6 CsCl6Br6 - 22.56 - 7 CsCl7Br5 - 19.34 - 8 CsCl8Br4 - 12.09 - 9 CsCl9Br3 - 5.37 - 10 CsCl10Br2 - 1.61 - 11 CsCl11Br1 - 0.29 - 12 CsCl12 100 0.02 -

m in CsBrmI12−m

CsBrmI12−m Calculated populations (%) Cs2SnBr6 Cs2SnBr4.5I1.5 Cs2SnBr3I3 Cs2SnBr1.5I4.5 Cs2SnI6

0 CsI12 - 0.00 0.02 3.17 100 1 CsBr1I11 - 0.00 0.29 12.67 - 2 CsBr2I10 - 0.00 1.61 23.23 - 3 CsBr3I9 - 0.04 5.37 25.81 - 4 CsBr4I8 - 0.23 12.09 19.36 - 5 CsBr5I7 - 1.15 19.34 10.32 - 6 CsBr6I6 - 4.02 22.56 4.02 - 7 CsBr7I5 - 10.32 19.34 1.15 - 8 CsBr8I4 - 19.36 12.09 0.23 - 9 CsBr9I3 - 25.81 5.37 0.04 - 10 CsBr10I2 - 23.23 1.61 0.00 - 11 CsBr11I1 - 12.67 0.29 0.00 - 12 CsBr12 100 3.17 0.02 0.00 -

S10

Table S8. Lorentzian profile fitted 133Cs MAS NMR spectra for CsBrmI12−m environments in Cs2SnBr1.5I4.5.

CsBrmI12−m Calculated binomial population (%)

Experimental (fitted 133Cs NMR spectra) 133Cs T1 (s) Area% (± 3) 𝛿iso (ppm) fwhm (Hz)

CsI12 3.17 9.3 -38.4 499 39.9 ± 4.5 CsBr1I11 12.67 18.1 -19.0 420 33.0 ± 3.5 CsBr2I10 23.23 22.9 -1.5 529 27.9 ± 3.0 CsBr3I9 25.81 23.0 14.9 757 23.7 ± 2.7 CsBr4I8 19.36 19.0 30.7 1125 19.8 ± 2.3 CsBr5I7 10.32 7.7 47.5 1195 14.2 ± 2.0

Table S9. Solid-state 133Cs and 119Sn NMR experimental parameters.

Table S9a. Solid-state 133Cs MAS NMR experimental parameters as discussed in Figure 2a in the main text. The 133Cs NMR spectra were acquired at 11.75 T using a 1.38 µs (5.5 µs π/2) Bloch pulse sequence.

Samples νrot (kHz) Recycle delay (s) # of scans Acquisition time (min) Cs2SnCl6

5.0

1200 4 80.0

Cs2SnBr6 500 4 33.3

Cs2SnI6 300 4 20.0

Table S9b. Solid-state 119Sn MAS NMR experimental parameters as discussed in Figure 2b in the main text. The 119Sn NMR spectra were acquired at 11.75 T with 13 kHz MAS using a 4.0 µs π/2 Hahn-echo pulse sequence. Samples νrot (kHz) Echo delay (μs) Recycle delay (s) # of scans Acquisition time (min)

Cs2SnCl6

10.0

70.9

200 32 107

Cs2SnBr6 15 512 128

Cs2SnI6 1 1024 17

S11

Table S9c. Solid-state 133Cs MAS NMR experimental parameters as discussed in Figure 6a in the main text. The 133Cs NMR spectra were acquired at 11.75 T using a 1.38 µs (5.5 µs π/2) Bloch pulse sequence.

Samples νrot (kHz) Recycle delay (s) # of scans Acquisition time (min)

Cs2SnCl6

13.0

1900 4 128

Cs2SnCl3Br3 1900 30 950

Cs2SnBr6 850 4 57

Cs2SnBr4.5I1.5 600 8 80

Cs2SnBr3I3 850 8 113

Cs2SnBr1.5I4.5 850 16 227

Cs2SnI6 450 4 30

Table S9d. Solid-state 119Sn NMR experimental parameters as discussed in Figure 7 in the main text. The 119Sn NMR spectra were acquired at 7.05 T using either 4.0 µs (62.5 kHz) or 1.7 µs (147.1 kHz) π/2 pulse in a Hahn-echo pulse sequence.

Samples νrot (kHz)

νrf (kHz) Echo delay (μs)

Recycle delay (s)

# of scans Acquisition time (min)

Cs2SnCl6

0.0

62.5 24.0 200 64 213

Cs2SnCl3Br3 147.1 27.0 210 768 2688

Cs2SnBr6 62.5 24.0 15 256 64

Cs2SnBr4.5I1.5 147.1 27.5 15 5120 × 2 offsets 1280 / offset

Cs2SnBr3I3 147.1 27.5 15 5120 × 3 offsets 1280 / offset

Cs2SnBr1.5I4.5 147.1 27.5 15 6144 × 2 offsets 1536 / offset

Cs2SnI6 62.5 24.0 0.2 15360 54

Table S9e. Solid-state 133Cs MAS NMR experimental parameters as discussed in Figure 9a in the main text. The 133Cs NMR spectra were acquired at 11.75 T using a 1.38 µs (5.5 µs π/2) Bloch pulse sequence.

Samples νrot (kHz) Recycle delay (s) # of scans Acquisition time (min)

BM-Cs2SnI6

5.0

120 4 8

HG-Cs2SnI6 120 16 32

SS-Cs2SnI6 120 16 32

S12

Table S9f. Solid-state 119Sn NMR experimental parameters as discussed in Figure 9b in the main text. The 119Sn NMR spectra were acquired at 7.05 T using a 4.0 µs π/2 Hahn-echo pulse sequence.

Samples νrot (kHz) Echo delay (μs) Recycle delay (s) # of scans Acquisition time (min) BM-Cs2SnI6

0.0

24.0

0.05 10240 9

HG-Cs2SnI6 0.2 10240 34

SS-Cs2SnI6 1.5 20480 512

Table S10. DFT computed NMR shielding parameters for structurally optimized SnClnBr6−n and SnBrnI6−n (n = 0−6) octahedral species determined using PBE0/TZ2P, incorporating relativistic effects with ZORA and spin-orbit coupling. Optimized structures are available in Table S10.

Octahedral species

Isotropic magnetic shielding, σiso (ppm)

Isotropic chemical shift, 𝛿iso (ppm)a

Span, Ω (ppm) Skew, κ

SnCl6 3828.6 -813.3 0.0 1.0 SnCl5Br 4040.6 -1025.9 375.2 1.0

cis-SnCl4Br2 4282.0 -1268.1 451.5 -0.95 trans-SnCl4Br2 4247.5 -1233.5 748.7 1.0 fac-SnCl3Br3 4550.7 -1537.6 20.6 -1.0 mer-SnCl3Br3 4517.2 -1504.0 895.8 -0.17 cis-SnCl2Br4 4812.0 -1799.7 458.6 0.93

trans-SnCl2Br4 4780.4 -1768.0 1058.3 -1.0 SnClBr5 5098.6 -2087.1 537.1 -1.0 SnBr6 5407.8 -2397.3 0.0 0.05 SnBr5I 5927.2 -2918.3 1294.0 1.0

cis-SnBr4I2 6461.9 -3454.6 1763.9 -0.97 trans-SnBr4I2 6426.5 -3419.1 2616.8 1.0 fac-SnBr3I3 6971.1 -3965.3 48.8 -1.0 mer-SnBr3I3 6965.1 -3959.4 3579.8 -0.26 cis-SnBr2I4 7440.4 -4436.0 1839.3 0.96

trans-SnI4Br2 7461.3 -4457.0 4702.2 -1.0 SnBrI5 7867.2 -4864.1 2426.9 -1.0 SnI6 8219.3 -5217.3 0.2 0.01

a Referenced with SnMe4 (σiso = 3017.74 ppm) at the same level of theory.

δiso = /"#$(012101341,667)./"#$(8+7691,667)%.{/"#$(012101341,667)×%&%&}

S13

Table S11. Optimized Cartesian coordinates of SnClnBr6−n and SnBrnI6−n (n = 0−6) octahedral species described in Table S11.

SnBr6 SnCl6 Atom X Y Z Atom X Y Z

Sn -0.000003 -0.000003 -0.000003 Sn -0.000001 -0.000001 -0.000001 Br 2.639876 0.000003 0.000003 Cl 2.469410 0.000001 0.000001 Br 0.000003 2.639881 0.000003 Cl 0.000001 2.469411 0.000001 Br 0.000003 0.000003 2.639886 Cl 0.000001 0.000001 2.469412 Br -2.639887 0.000003 0.000003 Cl -2.469413 0.000001 0.000001 Br 0.000003 -2.639892 0.000003 Cl 0.000001 -2.469414 0.000001 Br 0.000003 0.000003 -2.639897 Cl 0.000001 0.000001 -2.469415

SnI6 SnClBr5 Atom X Y Z Atom X Y Z

Sn -0.000004 -0.000004 -0.000005 Sn -0.000003 -0.000003 -0.018109 I 2.893406 0.000005 0.000005 Br 2.641710 0.000004 -0.013676 I 0.000005 2.893414 0.000005 Br 0.000004 2.641715 -0.013664 I 0.000005 0.000005 2.893421 Br 0.000004 0.000002 2.626767 I -2.893422 0.000005 0.000005 Br -2.641721 0.000004 -0.013674 I 0.000005 -2.893429 0.000005 Br 0.000004 -2.641726 -0.013666 I 0.000005 0.000005 -2.893437 Cl 0.000003 0.000004 -2.475419

SnCl5Br SnBrI5 Atom X Y Z Atom X Y Z

Sn -0.000001 -0.000001 0.022119 Sn -0.000004 -0.000004 -0.001161 Cl 2.466313 -0.000002 0.012272 I 2.894896 0.000008 0.004809 Cl 0.000003 2.466314 0.012272 I 0.000007 2.894905 0.004829 Cl 0.000000 0.000002 2.486203 I 0.000002 0.000001 2.901733 Cl -2.466315 -0.000002 0.012274 I -2.894912 0.000008 0.004811 Cl 0.000003 -2.466317 0.012271 I 0.000007 -2.894921 0.004831 Br -0.000003 0.000005 -2.635972 Br 0.000005 0.000004 -2.626872

SnBr5I cis-SnCl4Br2 Atom X Y Z Atom X Y Z

Sn -0.000004 -0.000004 0.008609 Sn -0.000001 0.020988 0.020987 Br 2.636476 0.000003 -0.007286 Cl 2.464143 0.011742 0.011732 Br 0.000003 2.636481 -0.007288 Cl -0.000002 2.482497 0.012951 Br 0.000004 0.000003 2.638473 Cl 0.000004 0.012945 2.482498 Br -2.636488 0.000003 -0.007281 Cl -2.464145 0.011735 0.011738 Br 0.000003 -2.636493 -0.007286 Br 0.000004 -2.632555 0.014093 I 0.000005 0.000007 -2.910922 Br -0.000002 0.014087 -2.632560

trans-SnCl4Br2 fac-SnCl3Br3 Atom X Y Z Atom X Y Z

Sn -0.000000 -0.000000 0.000000 Sn 0.019967 -0.019970 -0.019969 Cl 2.463699 0.000002 0.000000 Cl 2.479708 -0.012417 -0.012410 Cl 0.000002 2.463700 -0.000000 Br 0.013551 2.630149 -0.013546 Br -0.000001 -0.000001 2.650822 Br 0.013540 -0.013549 2.630154 Cl -2.463700 0.000002 0.000000 Br -2.630159 -0.013529 -0.013529 Cl 0.000002 -2.463701 -0.000000 Cl 0.012417 -2.479710 -0.012428 Br -0.000001 -0.000001 -2.650822 Cl 0.012417 -0.012414 -2.479711

S14

mer-SnCl3Br3 cis-SnCl2Br4 Atom X Y Z Atom X Y Z

Sn -0.000001 -0.000000 -0.019922 Sn -0.000003 -0.018998 -0.018998 Cl -0.000005 2.462300 -0.011247 Br 2.644252 -0.014176 -0.014179 Br 2.647077 0.000000 -0.014742 Br 0.000001 2.628099 -0.013039 Br 0.000009 0.000000 2.629670 Br 0.000006 -0.013034 2.628104 Br -2.647083 -0.000000 -0.014744 Br -2.644263 -0.014181 -0.014173 Cl -0.000005 -2.462300 -0.011247 Cl 0.000006 -2.477216 -0.011938 Cl 0.000008 -0.000000 -2.479209 Cl 0.000001 -0.011933 -2.477217

trans-SnCl2Br4 cis-SnBr4I2 Atom X Y Z Atom X Y Z

Sn -0.000002 -0.000002 0.000000 Sn -0.000004 0.006369 0.006370 Br 2.643734 0.000009 -0.000000 Br 2.634721 -0.008371 -0.008373 Br 0.000009 2.643739 0.000000 Br 0.000003 2.633869 -0.006290 Cl -0.000005 -0.000005 2.461125 Br 0.000003 -0.006290 2.633877 Br -2.643740 0.000009 -0.000000 Br -2.634734 -0.008370 -0.008368 Br 0.000009 -2.643745 0.000000 I 0.000007 -2.906641 -0.003545 Cl -0.000005 -0.000005 -2.461125 I 0.000005 -0.003545 -2.906651

trans-SnBr4I2 fac-SnBr3I3 Atom X Y Z Atom X Y Z

Sn -0.000002 -0.000002 -0.000000 Sn 0.004533 -0.004539 -0.004541 Br 2.633990 0.000008 0.000000 Br 2.630937 0.007380 0.007376 Br 0.000008 2.633995 0.000000 I -0.004723 2.904260 0.004719 I -0.000005 -0.000005 2.904931 I -0.004738 0.004719 2.904268

Br -2.633995 0.000008 -0.000000 I -2.904277 0.004752 0.004755 Br 0.000008 -2.634000 -0.000000 Br -0.007351 -2.630945 0.007355 I -0.000005 -0.000005 -2.904931 Br -0.007361 0.007353 -2.630953

mer-SnBr3I3 cis-SnBr2I4 Atom X Y Z Atom X Y Z

Sn -0.000002 -0.000000 -0.004261 Sn -0.000004 -0.002737 -0.002738 I 2.900318 0.000000 0.002595 I 2.897433 0.003773 0.003770

Br -0.000007 2.633482 0.009317 I 0.000003 2.902460 0.005789 I 0.000014 0.000000 2.903430 I 0.000004 0.005786 2.902467 I -2.900328 0.000000 0.002592 I -2.897450 0.003774 0.003773

Br -0.000007 -2.633482 0.009316 Br 0.000007 -2.628388 0.008315 Br 0.000012 0.000000 -2.630009 Br 0.000006 0.008312 -2.628396

trans-SnBr2I4 Atom X Y Z

Sn -0.000003 -0.000003 -0.000000 I 2.896449 0.000014 0.000000 I 0.000014 2.896458 -0.000000

Br -0.000007 -0.000008 2.633178 I -2.896459 0.000014 0.000000 I 0.000014 -2.896469 -0.000000

Br -0.000007 -0.000007 -2.633178

S15

Table S12. Calculated population distribution (binomial) for SnClnBr6−n and SnBrnI6−n environments in Cs2SnClxBr6−x and Cs2SnBrxI6−x (0 ≤ x ≤ 6).

Cs2SnClxBr6-x Calculated population of SnClnBr6−n (%)

SnCl6 SnCl5Br SnCl4Br2 (cis + trans)

SnCl3Br3 (fac + mer)

SnCl2Br4 (cis + trans)

SnClBr5 SnBr6

Cs2SnCl6 100 - - - - - - Cs2SnCl3Br3 1.56 9.38 23.44 31.25 23.44 9.38 1.56

Cs2SnBr6 - - - - - - 100 Cs2SnBrxI6-x Calculated population of SnBrnI6−n (%)

SnBr6 SnBr5I SnBr4I2 (cis + trans)

SnBr3I3 (fac + mer)

SnBr2I4 (cis + trans)

SnBrI5 SnI6

Cs2SnBr6 100 - - - - - - Cs2SnBr4.5I1.5 17.80 35.60 29.66 13.18 3.30 0.44 0.02 Cs2SnBr3I3 1.56 9.38 23.44 31.25 23.44 9.38 1.56

Cs2SnBr1.5I4.5 0.02 0.44 3.30 13.18 29.66 35.60 17.80 Cs2SnI6 - - - - - - 100

S16

Figure S1. Elemental analysis via energy dispersive X-ray spectroscopy for the solvent synthesized Cs2SnX6 (X = Cl, Br, I) parents as indicated.

S17

Figure S2. FESEM images for the solvent synthesized Cs2SnX6 (X = Cl, Br, I) parents.

Figure S3. Tauc plots showing direct bandgaps of the solvent synthesized Cs2SnX6 (X = Cl, Br, I) parents.

Figure S4. Solid-state 133Cs NMR spectra (frequency scale) of the solvent synthesized Cs2SnX6 (X = Cl, Br, I) parents at 11.75 T and νr = 5 kHz. The expansion shows the corresponding spinning side band.

S18

Figure S5. Solid-state 119Sn NMR spectra (frequency scale) of solvent synthesized Cs2SnBr6 at 7.05 and 11.75 T under MAS (νr = 10 kHz) conditions. The scales are vertically normalized.

Figure S6. Solid-state 119Sn NMR spectra (frequency scale) of solvent synthesized Cs2SnBr6 at 7.05 T under non-spinning and MAS (νr = 10 kHz) conditions. The scales are vertically normalized.

S19

Figure S7. Solid-state 35Cl NMR subspectra with different frequency offsets (250 kHz step size) acquired at 11.75 T using a frequency-stepped WURST-CPMG pulse sequence. Each spectrum was acquired using the following parameters: recycle delay = 1 s, numbers of scans = 512, numbers of echo = 50, spikelet = 5 kHz.

S20

Figure S8. Temperature dependent 119Sn NMR spectra (νr = 0 kHz, B0 = 11.75 T) for solvent synthesized

Cs2SnI6 sample.

Figure S9. Powder XRD patterns of ball-mill and hand-grind synthesized Cs2SnI6 materials at different grinding times as indicated. The asterisk (*) indicates unreacted CsI present in the final hand-ground product.

S21

Figure S10. Powder XRD patterns for ball mill synthesized Cs2SnBr6 materials with different CsBr and SnBr4 molar ratios. The ball milling times are 1.5 and 3.0 h for top and bottom diffractions, respectively. The asterisks (*) indicate excess CsBr present in the final product.

Figure S11. Powder XRD patterns of (a) Cs2SnCl3Br3 and (b) Cs2SnBr3I3 materials after 1.5 and 3.0 h of ball milling.

S22

Figure S12. FESEM image and the corresponding EDS elemental mapping for Cs, Sn, Cl, Br and I for the ball-milled Cs2SnCl3Br3, Cs2SnBr6, Cs2SnBr4.5I1.5 and Cs2SnBr1.5I4.5 samples as indicated.

S23

S24

Figure S13. Elemental analysis via energy dispersive X-ray spectroscopy for the ball-milled Cs2SnClxBr6−x

(x = 3) and Cs2SnBrxI6−x (x = 0, 1.5, 3, 4.5, 6) samples as indicated.

S25

Figure S14. The experimental and fitted powder XRD diagrams of ball-milled Cs2SnClxBr6−x (x = 3) and Cs2SnBrxI6−x (x = 0, 1.5, 3, 4.5, 6) samples. All the diffraction data are fitted assuming a cubic (Fm3'm) space group symmetry.

S26

Figure S15. Tauc plots showing direct bandgaps of ball-milled Cs2SnClxBr6−x (x = 3) and Cs2SnBrxI6−x (x = 0, 1.5, 3, 4.5, 6) samples.

S27

Figure S16. Calculated population distributions (binomial) of local cuboctahedral environments: (a) CsClmBr12−m in Cs2SnClxBr6−x (x = 0, 3, 6) and (b) CsBrmI12−m in Cs2SnBrxI6−x (x = 0, 1.5, 3, 4.5, 6).

S28

Figure S17. Solid-state 133Cs MAS NMR spectra of Cs2SnCl6, Cs2SnCl3Br3 and Cs2SnBr6 at 11.75 T with a MAS frequency of 13 kHz. The expansion shows the corresponding spinning side band.

Figure S18. Normalized 133Cs NMR spectra (B0 = 11.75 T, νrot = 13 kHz) exhibiting spectral overlap of multiple resonances with recycle delays of 10 and 850 s for Cs2SnBr1.5I4.5. The 133Cs NMR spectra were acquired using a short tip angle (~15o) Bloch pulse sequence.

S29

Figure S19. DFT computed results for SnBrnI6−n (n = 0−6) environments: (a) optimized geometries and (b) 119Sn isotropic chemical shift as a function of the number of attached Br atoms in SnBrnI6−n.

Figure S20. Simulated 119Sn NMR spectra for SnBrnI6−n (n = 0−6) environments. The anisotropic parameters were obtained from DFT computation.

S30

Figure S21. Calculated population distributions (binomial) for local octahedral environments: (a) SnClnBr6−n in Cs2SnClxBr6−x (x = 0, 3, 6) and (b) SnBrnI6−n in Cs2SnBrxI6−x (x = 0, 1.5, 3, 4.5, 6).

S31

Figure S22. Powder XRD patterns of Cs2SnI6 samples prepared by ball-milling (BM) and solvent-synthesis (SS) routes.

S32

REFERENCES

(1) Nairui, X.; Yehua, T.; Yali, Q.; Duoduo, L.; Ke-Fan, W. One-Step Solution Synthesis and Stability Study of Inorganic Perovskite Semiconductor Cs2SnI6. Sol. Energy 2020, 204, 429–439.

(2) Kubelka, P.; Munk, F. Ein Beitrag Zur Optik Der Farbanstriche. Z.Tech. Phys. (Leipzig) 1931, 12, 593–601.

(3) Bernard, G. M.; Goyal, A.; Miskolzie, M.; McKay, R.; Wu, Q.; Wasylishen, R. E.; Michaelis, V. K. Methylammonium Lead Chloride: A Sensitive Sample for an Accurate NMR Thermometer. J. Magn. Reson. 2017, 283, 14–21.

(4) O’Dell, L. A.; Schurko, R. W. QCPMG Using Adiabatic Pulses for Faster Acquisition of Ultra-Wideline NMR Spectra. Chem. Phys. Lett. 2008, 464, 97–102.

(5) Dell, L. A. O. The WURST Kind of Pulses in Solid-State NMR. Solid State Nucl. Magn. Reson. 2013, 55–56, 28–41.

(6) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra, C.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. Chemistry with ADF. J. Comput. Chem. 2001, 22 (9), 931–967.

(7) Guerra, C. F.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Towards an Order-N DFT Method. Theor. Chem. Acc. 1998, 99, 391–403.

(8) Baerends, E. J.; Ziegler, T.; Atkins, A. J.; Autschbach, J.; Bashford, D.; Baseggio, O.; Bérces, A.; Bickelhaupt, F. M.; Bo, C.; Boerritger, P. M.; et al. ADF2017, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands, https://www.scm.com.

(9) Adamo, C.; Barone, V. Toward Reliable Density Functional Methods without Adjustable Parameters: The PBE0 Model. J. Chem. Phys. 1999, 110 (13), 6158–6170.

(10) Lenthe, E. van; Baerends, E. J. Optimized Slater-Type Basis Sets for the Elements 1-118. J. Comput. Chem. 2003, 24 (9), 1142–1156.

(11) Wolff, S. K.; Ziegler, T.; Lenthe, E. van; Baerends, E. J. Density Functional Calculations of Nuclear Magnetic Shieldings Using the Zeroth-Order Regular Approximation (ZORA) for Relativistic Effects: ZORA Nuclear Magnetic Resonance. J. Chem. Phys. 1999, 110 (16), 7689–7698.

(12) Lenthe, E. van; Baerends, E. J.; Snijders, J. G. Relativistic Regular Two‐component Hamiltonians. J. Chem. Phys. 1993, 99, 4597–4610.

(13) Autschbach, J. The Role of the Exchange-Correlation Response Kernel and Scaling Corrections in Relativistic Density Functional Nuclear Magnetic Shielding Calculations with the Zeroth-Order Regular Approximation. Mol. Phys. 2013, 111, 2544–2554.

(14) Wolff, S. K.; Ziegler, T. Calculation of DFT-GIAO NMR Shifts with the Inclusion of Spin-Orbit Coupling. J. Chem. Phys. 1998, 109 (3), 895–905.

(15) Harris, R. K.; Becker, E. D. NMR Nomenclature: Nuclear Spin Properties and Conventions for Chemical Shifts—IUPAC Recommendations. J. Magn. Reson. 2002, 156, 323–326.

download fileview on ChemRxivCs2SnX6_SI.pdf (6.02 MiB)

Uncovering Halide Mixing and Octahedral Dynamicsin Vacancy-Ordered Double Perovskites by Multinuclear Magnetic Resonance SpectroscopyAbhoy Karmakar, Srija Mukhopadhyay, Pierre G.B. Gachod, Victor Arturo Mora-Gomez, Guy M. Bernard, Alex Brown, and Vladimir K. Michaelis*

Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada

ABSTRACT: Vacancy-ordered double perovskites Cs2SnX6 (X = Cl, Br, I) have emerged as promisinglead-free and ambient-stable materials for photovoltaic and optoelectronic applications. To advancethese promising materials, it is crucial to determine the correlations between physical properties andtheir local structure and dynamics. Solid-state NMR spectroscopy of multiple NMR-active nuclei (133Cs,119Sn and 35Cl) in these cesium tin(IV) halides has been used to decode the structure, which plays a keyrole in the materials’ optical properties. The 119Sn NMR chemical shifts span approximately 4000 ppmand the 119Sn spin-lattice relaxation times span three orders of magnitude when the halogen goes fromchlorine to iodine in these diamagnetic compounds. Moreover, ultrawideline 35Cl NMR spectroscopy forCs2SnCl6 indicates an axially symmetric chlorine electric field gradient tensor with a large quadrupolarcoupling constant of ca. 32 MHz, suggesting a chlorine that is directly attached to Sn(IV) ions. Variabletemperature 119Sn spin lattice relaxation time measurements uncover the presence of hidden dynamicsof octahedral SnI6 units in Cs2SnI6 with a low activation energy barrier of 12.45 kJ/mol (0.129 eV). Wefurther show that complete mixed-halide solid solutions of Cs2SnClxBr6−x and Cs2SnBrxI6−x (0 ≤ x ≤ 6) format any halogen compositional ratio. 119Sn and 133Cs NMR spectroscopy resolve the unique local SnClnBr6−n

and SnBrnI6−n (n = 0−6) octahedral and CsBrmI12−m (m = 0−12) cuboctahedral environments in the mixed-halide samples. The experimentally observed 119Sn NMR results are consistent with magnetic shieldingparameters obtained by density functional theory computations to verify random halogen distribution inmixed-halide analogues. Finally, we demonstrate the difference in the local structures and opticalabsorption properties of Cs2SnI6 samples prepared by solvent-assisted and solvent-free synthesis routes.

INTRODUCTION Lead halide perovskites (APbX3; A = Cs+, CH3NH3

+,CH(NH2)2

+; X = Cl−, Br−, I−) have gainedtremendous attention as semiconductingmaterials due to their desirable optical andelectrical properties including in photovoltaicsolar cells,1 light-emitting diodes,2 hard-radiationdetectors,3 photocatalytic reactions4 andbeyond.5,6 Although lead-containing perovskiteshave high power conversion efficiencies (PCE) of25.5% in photovoltaic solar cells,7 they,unfortunately, suffer from mediocre chemicalstability8–11 and the potential risk of leadtoxicity.12,13 The tin(II) halide perovskites ASnX3

are possible alternatives with lower toxicity, butthey exhibit lower PCE of up to 10% and rapidlyoxidize in air (Sn2+ to Sn4+).14–17

Vacancy-ordered double perovskites A2BX6 (A =Cs+, CH3NH3

+, CH(NH2)2+; B = Sn4+; X = Cl−, Br−,

I−) are an alternative family of perovskites,demonstrating much higher ambient stability andlower toxicity.18 The structure of A2BX6 can bedescribed as an ABX3 perovskite, with half of the

B-sites removed in an ordered fashion (Figure1a).19 For example, Cs2SnX6 are cubic in structurewith 12 halogens surrounding the A-site to form aCsX12 cuboctahedron, whereas the B-site isoctahedrally coordinated with six halogen anionsto form BX6 octahedra (Figure 1a). The iodideanalogue, Cs2SnI6, has a bandgap (1.25−1.62 eV)close to the optimum value (1.34 eV) to maximizethin-film solar cell efficiency.18,20,21 Recently,Cs2SnI6 has shown promising optoelectronicproperties to develop air-stable and lead-freeperovskite photovoltaics.22 The mixed-halideanalogues Cs2SnBrxI6−x reveal bandgaptailorability with halogen composition,23 makingthem suitable as hole-transporting materials insolid-state dye-sensitized solar cells.24 On theother hand, Cs2SnX6 nanocrystals havedemonstrated tunable light emission andphotocatalytic activity.25–27 Recently, doped (Bi3+

and Sb3+) Cs2SnCl6 has been shown to produceefficient blue and orange-red light emission; thisdevelopment brings about de novo opportunitiesfor these materials as solid-state lightingcandidates.28–30

To date, vacancy-ordered halide doubleperovskites have been extensively characterizedby X-ray diffraction (XRD) techniques, whichprovide information about the average long-rangestructure, but not about the local chemicalenvironments. Fundamentally, it is essential tocorrelate the bulk photophysical properties withthe microstructural characteristics forperovskites. Solid-state nuclear magneticresonance (NMR) spectroscopy is a robustanalytical characterization tool to determineshort- (<5 Å) and medium- (5−10 Å) rangestructures as well as ion dynamics inperovskites.16,31–47 More specifically, 133Cs (I = 7/2,Qm = −0.34 fm2, 100% abundance)48 is an idealNMR-active nucleus to identify the chemicalenvironments around the A-site inperovskites.36,42,47,49–52 119Sn (I = 1/2, 8.6%abundance), the most receptive nucleus amongthree NMR-active tin isotopes (115Sn, 117Sn, 119Sn)(Table S1),48 has been used to resolve the local B-site structural environments and halogendynamics in ABX3 perovskites and other tin-containing compounds.16,17,53–59 Furthermore, the119Sn NMR spectra displays an extensive range todiamagnetic Sn-containing compounds which hasbeen recently extended to span nearly 6,000ppm. 16,60–62

This study shows an impressive ability to tailorthe optical bandgap over a 3 eV range using ahigh energy mechanochemical synthetic designof vacancy ordered double perovskiteCs2SnClxBr6−x and Cs2SnBrxI6−x mixed-halidematerials. Powder XRD and NMR spectroscopyallow identification of the long-, medium- andshort-range chemical and coordinationenvironments around Cs and Sn sites uponinterhalogen substitutions in mixed-halideanalogues. The experimentally observed broad(>100 kHz) and multiple resonances in the 119SnNMR spectra for the mixed-halide analogues arecorrelated with 119Sn magnetic shieldingparameters obtained from density functionaltheory (DFT) computations and reveal theimportance of the spin-orbit shielding term.Variable temperature 119Sn NMR captures theactivation energy of the rapid SnI6 octahedraldynamics in Cs2SnI6. Finally, Cs2SnI6 materialsprepared by solvent-assisted or solvent-freesynthesis routes are discussed to uncover theinfluences of solvent- vs. solvent-free synthesison the local chemical structure and bulk opticalproperties. The comparison is important asmechanochemistry continues to be shown as aneffective greener approach for synthesizingsolids.

Figure 1. (a) Crystal structure of Cs2SnX6, localCsX12 cuboctahedral environment of 12 halidesaround cesium, and local SnX6 octahedralenvironment of six halides around tin. (b) Opticalabsorption spectra (inset – photographs) and (c)Powder XRD patterns of solvent synthesized Cs2SnX6

parents.

Table 1. Unit cell parameters, direct bandgap, solid-state 133Cs and 119Sn NMR results (νr = 13 kHz, B0 =11.75 T) for solvent synthesized Cs2SnX6 parents.

Figure 2. Solid-state (a) 133Cs, (b) 119Sn NMR spectra and (c) inversion recovery plot to measure 119Sn spin-lattice (T1) relaxation values for the solvent synthesized Cs2SnX6 parents. NMR experiments were performed at11.75 T with magic-angle spinning frequency of 13 kHz.

RESULTS AND DISCUSSIONSolvent Synthesized Cs2SnX6 Parents.Cs2SnX6 (X = Cl−, Br−, I−) parents were preparedby solvent-assisted synthetic routes starting withCsX and SnX4 precursors (see SI). As determinedby energy-dispersive X-ray spectroscopy (EDS),the chemical compositions agree well with theexpected Cs2SnX6 chemical formulation formed bysolvent synthesis (Table S2 and Figure S1).Cs2SnX6 samples are polycrystalline with micron-sized particles as determined by field-emissionscanning electron microscopy (FESEM) (FigureS2). The color of Cs2SnX6 changes from white(Cs2SnCl6) to faint yellow (Cs2SnBr6) to black(Cs2SnI6), consistent with a decrease in theiroptical absorption edge (Figure 1b). The powderXRD patterns are consistent with a cubicstructure (K2PtCl6-type, space group Fm3m) for allthree Cs2SnX6 parents at room temperature(Figure 1c). The cell parameters, as refined by LeBail fitting of the powder XRD patterns,systematically increase for the Cs2SnX6 parents asthe ionic radius of the associated halogen anionincreases from 1.81 Å r(Cl–) < 1.96 Å r(Br–) < 2.20Å r(I–) (Table 1).63

The optical absorption spectra, which areextracted from the diffuse reflectance spectra bythe Kubelka-Munk function, exhibit a gradual shiftof the absorption edge to lower energy as Cl isreplaced by Br and followed by I in Cs2SnX6

parents. Considering a direct bandgap propertyfor these semiconducting materials,64 the optical

bandgap values are extracted from the linearregion in the Tauc plot of (αhν)2 against energy(Figure S3). The bandgap in Cs2SnX6 parentsexhibits a dramatic decrease from 4.48 eV forCs2SnCl6 to 1.42 eV for Cs2SnI6, spanning across 3eV (Table 1).

Solid-state 133Cs and 119Sn NMR spectroscopy wasused to investigate the local halogenenvironments around cesium and tin. The Csatoms occupy a crystallographic site with 4 3msymmetry in Cs2SnX6. Twelve nearest-neighboringhalogen atoms surround them to form a localCsX12 cuboctahedral environment (Figure 1a).Accordingly, the 133Cs magic-angle spinning (MAS)NMR spectra exhibit a single sharp resonancewith chemical shifts of 125.8 ppm (full width athalf maximum, fwhm = 62 Hz) for Cs2SnCl6, 114.6ppm (fwhm = 175 Hz) for Cs2SnBr6 and −41.4ppm (fwhm = 66 Hz) for Cs2SnI6, indicating oneunique crystallographic Cs site (Figure 2a andTable 1). A gradual change in the 133Cs NMRchemical shift towards lower frequency is inagreement with a recent observation on CsSnX3

perovskites,16 but in contrast to CsPbX3

perovskites,36 where an opposite trend in thechemical shift was observed. Due to Cs residing inthe center of a perfect CsX12 cuboctahedralenvironment in Cs2SnX6 parents, a vanishing 133Csquadrupole coupling constant (CQ = 0 kHz) isexpected with no spinning sidebands (SSBs)under MAS conditions. However, a low-intensity(<1%) SSB was observed under slow MAS (νr = 5kHz) for all three Cs2SnX6 parents (Figure S4),

Samples

Cellparameter

(Å)

Bandgap (eV)

133Cs NMR 119Sn NMR

δ(ppm) fwhm (Hz) δ(ppm) fwhm (kHz) T1 (s)

Cs2SnCl6

10.3826(1) 4.48 125.8 ± 0.1 62 ± 2 –708 ± 1 1.5 ± 0.1 146 ± 6

Cs2SnBr6

10.8377(2) 3.13 112.3 ± 0.2 175 ± 5 –1964 ± 2 5.2 ± 0.2 5.4 ± 0.2

Cs2SnI6 11.6342(2) 1.42 –46.1 ± 0.1 66 ± 2 –4540 ± 5 11.5 ± 0.5 0.170 ±0.007

which is associated to intrinsic defects such ashalogen vacancies.49,65

The Sn atoms occupy m3m site symmetry in theCs2SnX6 structure with six nearest-neighborhalogen atoms surrounding Sn to form local SnX6

octahedral units (Figure 1a). The Cs2SnX6 parentsexhibit symmetric 119Sn NMR resonances with noevidence of SSBs under MAS conditions (Figure2b), consistent with the highly symmetric localSnX6 octahedral environment. The 119Sn NMRspectra in Cs2SnX6 shift towards lower frequencywith a chemical shift of −708 ppm for Cs2SnCl6,−1964 ppm for Cs2SnBr6 and −4540 ppm forCs2SnI6, a large change spanning over 3800 ppmfor diamagnetic tin-containing compounds. Thetrend of increased Sn shielding (to lowerfrequency) with increased halogen atomicnumber observed here, a normal halogendependence (NHD), is opposite to previousstudies on Sn(II)-based perovskites (ASnX3),where an inverse halogen dependence (IHD) wasobserved with a smaller chemical shift range (<1300 ppm).16,56,66 The unprecedented change in119Sn chemical shift was further studied usingtheoretical calculations, which revealed that thediamagnetic and paramagnetic shielding termsare nearly unchanged. Hence, the changes inchemical shift originate from considerable spin-orbit effects whereby this term changes ca. 4500ppm as the halide anions increase in size (TableS3), emphasizing the need to consider relativisticand spin-orbit effects when performing DFTcalculations for these materials which containheavy elements. The 119Sn NMR linewidthsdepend on the halogen in Cs2SnX6, increasingmonotonically from 1.5 (Cs2SnCl6) to 5.2(Cs2SnBr6) to 11.5 kHz (Cs2SnI6). For Cs2SnBr6, the119Sn NMR linewidth and lineshape areindependent of the magnetic field strength (B0 =7.05 to 11.75 T, Figure S5), indicating noevidence of shielding anisotropy. Furthermore, the119Sn NMR linewidth decreases slightly (ca. 0.8kHz) once MAS is applied (Figure S6), confirminga small contribution from heteronuclear dipolarcoupling (through space) between Sn and Brnuclei (79/81Br: I = 3/2, total abundance = 100%).These findings suggest that a dominant1J(119Sn−79/81Br) scalar coupling between 119Sn andthe six directly bonded 79/81Br nuclei in SnBr6

octahedra is responsible for the 119Sn NMRlinewidth.35,36,38,67 The 119Sn spin-lattice relaxationtime (T1) decreases from Cs2SnCl6 (T1 = 146 s) toCs2SnBr6 (T1 = 5.4 s) to Cs2SnI6 (T1 = 0.170 s), adifference of three orders of magnitude (Figure 2cand Table 1), which is comparable to recentobservation on ASnX3 perovskites.16,17 The trend in119Sn T1 values where the relaxation decreasesdramatically with increasing halide atomicnumber (Figure 2c) follows the inverse of thetrend in the scalar coupling strength where1J(119Sn−127I) > 1J(119Sn−79/81Br) > 1J(119Sn−35/37Cl).Hence this interaction is thought to be thedominant relaxation mechanism and expected to

be fastest for Cs2SnI6, intermediate for Cs2SnBr6

and slowest for Cs2SnCl6, as previously observedin other tin halide perovskite compounds.16,68,69

Chlorine-35 NMR spectroscopy in Cs2SnCl6.The stable halogen elements are NMR active, butunlike 19F (I = 1/2), the heavier group 17 elementsare quadrupolar (nuclear spin, I = 3/2 (35/37Cl,79/81Br) or I = 5/2 (127I)) with large quadrupolemoments (Table S1). 35Cl NMR spectroscopy hasbeen used to investigate the local structuralenvironments in pharmaceuticals, amino acids,lead halide perovskites, organic andorganometallic compounds, due to its overallNMR sensitivity as well as the insight intostructure that it provides.70–75 Crystallographically,the halogen atoms in Cs2SnX6 occupy a single sitewith symmetry 4m.m, where each halogen isbonded to a single Sn atom (Figure 1a). Hence,this lower site symmetry of the X site in Cs2SnX6

is expected to impart a sizable electric fieldgradient (EFG) that would cause substantialspectral broadening. Figure 3 shows thefrequency-stepped wide-band, uniform, andsmooth truncation Carr-Purcell-Meiboom-Gill(WURST−CPMG)76,77 35Cl NMR spectrum ofCs2SnCl6. The ultrawideline NMR spectrum thatspans ca. 2.7 MHz at 11.75 T was obtained in 12steps using the variable offset cumulative spectra(VOCS) technique (Figure S7). Fitting the centraltransition reveals that δiso = 560 ± 20 ppm, CQ =32.0 ± 0.3 MHz and that the quadrupolarasymmetry parameter, η = 0.00. The spectrallineshape is consistent with the presence of aterminal Cl atom in Cs2SnCl6, as the terminal Clatoms are in a higher axial symmetricenvironment, which results in a large CQ and a lowη. Moreover, the large 35Cl CQ value confirms thechloride ions are coordinated to a Sn(IV) ion in alocal SnCl6 octahedral environment.75 In contrast,Cl atoms attached to Sn(II) ions show lower CQ

values of <30 MHz.75 We note that bulk andnanocrystalline forms of CsPbCl3 perovskiteexhibit lower 35Cl CQ values of ca. 15.5 MHz (η =0).73

Figure 3. Experimental (lower trace, blue) andsimulated (upper trace, red) 35Cl WURST-CPMG solid-state NMR spectrum of Cs2SnCl6 parent (νr = 0 kHz,B0 = 11.75 T, sum of 12 frequency steps, 250kHz/step).

SnI6 Octahedral Dynamics in Cs2SnI6.Anharmonic lattice dynamics of BX6 octahedra inhalide perovskites play a vital role in optical andelectrical properties of these materials.78–80 Theisolated BX6 octahedral units in A2BX6 vacancy-ordered double perovskites extend the degree ofdynamical freedom compared to that for theirsister materials, ABX3 perovskites (Figure 4a).Recently, X-ray pair distribution function analysison Cs2SnI6 has revealed asymmetry in theinteroctahedral I−I distance, which can becorrelated with rotational disorder for the isolatedSnI6 octahedral units, but the technique does notprovide information about the activation energyassociated with this rotation.81,82 Therefore,variable-temperature 119Sn NMR measurementswere carried out on a solvent-synthesized Cs2SnI6sample to probe specifically the anharmonicity ofthe isolated octahedral SnI6 units to determinethe associated activation energies needed for thisunique property.

As the sample temperature increases from 208 to380 K, the 119Sn NMR resonance shifts linearlytowards lower frequencies from −4522 to −4547ppm (Table S4, Figures 4b and S8), attributed to achange in the interatomic distance as thetemperature changes. The linewidth of 119Sn NMRresonance remains nearly unaltered (fwhm = 18± 1 kHz) across the temperature ranges studied,indicating that the 119Sn NMR linewidth in Cs2SnI6is primarily driven by indirect (J) and direct(dipolar) spin-spin coupling between 119Sn and 127Inuclei. Most interestingly, the 119Sn NMR spin-lattice relaxation time (T1) changes drasticallyfrom 4.6 s (208 K) to 0.12 s (380 K); nearly a 40-fold change in the T1 over the 170 K temperaturerange. A plot of ln[T1/s] against [1000/T(K)] is

linear allowing a determination of an activationenergy of 12.45 ± 0.87 kJ/mol (0.129 ± 0.009eV), in accordance with the Arrhenius relationship(see Table S4, Figure 4b, Supplementary Note 1),suggesting a low energy barrier for the SnI6

octahedral lattice dynamics. A comparabletemperature dependent change in 119Sn T1 hasbeen reported for the ionic conductors,CH3NH3SnBr3 and CsSnBr3 perovskites, wheremuch higher activation energies (0.3−0.4 eV)were observed for rapid halogen migration.16,17

Figure 4. (a) Schematic diagram of SnI6

anharmonicity in Cs2SnI6 structure. (b) Variabletemperature 119Sn T1 data (νr = 0 kHz, B0 = 11.75 T)as a function of inverse of temperature for solventsynthesized Cs2SnI6.

Mechanochemically SynthesizedCs2SnClxBr6−x and Cs2SnBrxI6−x Mixed-Halides.Halide perovskites are predominately prepared bysolvent-assisted, high temperature or vapordeposition techniques.1,83,84 Mechanochemicalsynthesis is an entrenched “greener” syntheticroute to prepare a broad range of advancedfunctional materials,85–87 and has recently beenapplied successfully in large-scale production ofperovskite photovoltaic materials.36,38,88–92

Moreover, certain compositions of APbX3 mixed-halide perovskites were achieved only bymechanochemical synthesis, but not via solvent-assisted routes.37,38,93

Both manual hand-grinding using mortar andpestle and automated electric ball-milling havebeen used as solvent-free mechanochemicalsynthesis routes to prepare bulk halideperovskites.16,36–38,89,90 Recently, we have shownthat, using a hand-grinding approach, one can

obtain phase pure APbX3 perovskites withinhours.36–38 However, replacing lead(II) with tin(IV),we find that hand-grinding preparation of Cs2SnI6requires a substantially longer mixing time (6 h).In comparison, electric ball-milling is moreefficient (1.5 h) over the labor-intensive manualroute (Figure S9). The phase pure Cs2SnClxBr6−x (x= 3) and Cs2SnBrxI6−x (x = 0, 1.5, 3, 4.5, 6)samples were prepared by mechanochemical ball-milling under ambient conditions (detailedsynthesis is discussed in SI; see Experimental,Table S5 and Figures S10-S11).

Figure 5a shows photographs of the ball-milledCs2SnClxBr6−x and Cs2SnBrxI6−x samples and theirassociated colors consistent with the opticalbandgaps. The samples are micron-sized particleshaving uniform distributions of the constituentelements (Cs, Sn, Cl, Br, I), as determined fromelemental mapping images collected using FESEM(Figures 5b and S12). The elemental compositionsof mixed-halide analogues, as determined by EDSanalysis, agree well with the nominalcompositions (Table S2 and Figure S13).

The powder XRD patterns collected at roomtemperature are consistent with the cubicstructure (space group Fm3m) for Cs2SnClxBr6−x

and Cs2SnBrxI6−x (Figures 5c and S14). The powderXRD peaks are shifted towards a higherdiffraction angle (Figure 5c), and the refined unitcell parameter increases as smaller halide ionsare replaced with larger variants (Table S6). Theunit cell parameter in Cs2SnBrxI6−x against Brcontent shows no significant deviation fromlinearity, in agreement with Vegard’s law (Figure5e). This finding suggests that Br and I atoms arerandomly mixed in Cs2SnBrxI6−x; consistent withour previous observations in lead-based mixed-halide perovskites.37,38

The optical absorption spectra show that theabsorption edge shifts to lower energy withgreater Br content in Cs2SnClxBr6−x and I contentin Cs2SnBrxI6−x (Figure 5d). As extracted from theTauc plot of the direct bandgap, the opticalbandgap values span about 3 eV for thesesamples (Table S6 and Figure S15), which agreeswell with the previous observations.64,94 Thebandgap in the Cs2SnBrxI6−x mixed-halide seriesdecreases nonlinearly from 3.30 eV for Cs2SnBr6

to 1.54 eV for Cs2SnI6 (Figure 5e). This behavior isdifferent from the linear bandgap trend observedin lead-containing mixed-halide perovskites,36–38

but is similar to the observed anomalousnonlinear trend in the mixed tin-leadperovskites.95

A more complete local structural model for themixed-halide samples can be obtained by solid-state 133Cs and 119Sn NMR spectroscopy tointerrogate the A- and B-sites, respectively. Figure6a shows the 133Cs MAS NMR spectra forCs2SnClxBr6−x and Cs2SnBrxI6−x. Cs2SnCl3Br3 exhibitsan inhomogeneously broadened 133Cs NMRresonance (fwhm = 1.1 kHz) due to the spectral

overlap of multiple local CsClmBr12−m (m = 0−12)halide environments according to the randomdistribution (Table S7 and Figure S16a).36 The133Cs SSB manifold is related to the local CsX12

cuboctahedral symmetry of perovskites,49 anincrease in the 133Cs SSBs in Cs2SnCl3Br3 indicatesa perturbation within the electric field gradientdue to the lowering in the local cuboctahedralsymmetry upon Cl and Br mixing, and hence anincrease in the quadrupole coupling constant(Figure S17). For the Cs2SnBrxI6−x series, the 133CsNMR resonances span across 160 ppm between115 to −45 ppm. The resonances are eitherspectrally unresolved for Br-rich or resolved for I-rich samples (Figure 6a). The Br-rich sample,Cs2SnBr4.5I1.5, shows an inhomogeneouslybroadened 133Cs NMR peak due to spectraloverlapping of multiple local CsBrmI12−m

environments according to the random populationdistribution (Table S7 and Figure S16b), therebylimiting spectral resolution. Cs2SnBr3I3 whichcontains equal proportions of Br and I atoms,exhibits a partially spectrally resolved 133Cs NMRresonance. Remarkably, the I-rich sample,Cs2SnBr1.5I4.5 shows spectrally resolved 133Cs NMRresonances, which correspond to sixdistinguishably resolved local CsBrmI12−m (m = 0,1, 2, 3, 4, 5) environments (Figure 6b). The lowestchemical shift at −38.4 ppm (fwhm = 500 Hz) isassigned to the CsI12 environment, which iscomparable to the local chemical environment at−41.5 ppm (fwhm = 110 Hz) for Cs2SnI6 parent(Figure 6a). The observed broadening and shift ofthe resonance to higher frequency is associatedwith the lower site symmetry than cubic as thenext-nearest neighbors in the medium-rangestructure (>5 Å) consists of a mixture of Br and Iatoms.49 The remaining peaks within the sets of133Cs resonances in Cs2SnBr1.5I4.5 shift towardshigher frequency as I is substituted by Br (Figure6c and Table S8), following a strict lineardependence [Eq (1)] with m in CsBrmI12−m.

δ(133Cs)/ppm = 17.0∙m – 36.78 (1)

Furthermore, the observation of increasing 133Cslinewidth for CsBrmI12−m peaks with increased Brcontent (Table S8) suggests the spectral overlapof multiple positional isomers for uniqueCsBrmI12−m environments that consists of amixture of Br and I atoms in 12 positions. The133Cs spin-lattice relaxation times decreaselinearly from 40 to 14 s with increase in numberof Br attached in CsBrmI12−m (Figure 6c). Acquiringthe 133Cs NMR spectra with a quantitative recycledelay for Cs2SnBr1.5I4.5 (Figure S18), the fitted 133Cspeak areas for each CsBrmI12−m environmentclosely follow a binomial population distribution(Figure 6d and Table S8). This observation isconsistent with random Br and I mixing at thelocal structure, forming an atomic-level mixed-halide solid-solution without evidence of halogen-rich phase segregation or domains.

The 119Sn NMR spectra for the Cs2SnClxBr6−x andCs2SnBrxI6−x series are fascinating and, exhibitbroad (100s of kHz) and partial spectrallyresolved multiple 119Sn resonances (Figure 7). Toachieve maximum 119Sn NMR sensitivity,optimized 119Sn recycle delays and variable offsetcumulative spectra (if required) were used due totheir drastic differences in 119Sn T1 and largechemical shift spans, respectively (Table S9d).This observation contrasts with the recentobservations on ASnX3 mixed-halide perovskites,where single 119Sn resonances are reported due torapid halogen dynamics.16,17 However, thebroadening and multi resonance spectralbehavior agrees well with 207Pb NMR spectra forAPbX3 mixed-halide perovskites.36–38

Figure 5. (a) Photographs of ball-milled samples: (i) Cs2SnCl3Br3, (ii) Cs2SnBr6, (iii) Cs2SnBr1.5I4.5, (iv) Cs2SnBr3I3,(v) Cs2SnBr1.5I4.5 and (vi) Cs2SnI6. (b) Scanning electron micrograph and elemental mapping for Cs2SnBr3I3. (c)Powder XRD patterns, (d) normalized optical absorption spectra of ball-milled Cs2SnClxBr6−x and Cs2SnBrxI6−x

samples (except Cs2SnCl6, solvent synthesized). (e) Plots of unit cell parameters and bandgaps of ball-milledCs2SnBrxI6−x series.

Figure 6. (a) Solid-state 133Cs (νr = 13 kHz, B0 = 11.75 T) NMR spectra of ball-milled Cs2SnClxBr6−x andCs2SnBrxI6−x samples (except Cs2SnCl6, solvent synthesized). (b) Experimental and fitted 133Cs NMR spectra, (c)133Cs chemical shift and 133Cs fwhm values and (d) population distribution as a function of Br atoms inCsBrmI12−m for Cs2SnBr1.5I4.5.

Figure 7. Solid-state 119Sn (non-spinning, B0 = 7.05 T) NMR spectra of (a) ball-milled Cs2SnClxBr6−x andCs2SnBrxI6−x (except Cs2SnCl6, solvent synthesized), (b) Cs2SnCl3Br3 and (c) Cs2SnBr4.5I1.5. The local SnClnBr6−n

and SnBrnI6−n environments are assigned in (b) and (c).

To understand the observed 119Sn NMR spectra formixed-halide analogues, the 119Sn magneticshielding parameters of isolated octahedralclusters, i.e., SnClnBr6−n (n = 0−6) for Cs2SnClxBr6-x

and SnBrnI6−n for Cs2SnBrxI6-x were computed usingDFT and the computational details are provided inthe SI (see Experimental, Tables S10-S11). Thecomputed 119Sn isotropic chemical shifts arelinearly shifted to lower frequency as Cl issubstituted by Br in SnClnBr6−n (Figure 8a) and Bris substituted by I in SnBrnI6−n (Figure S19). Thecomputed magnetic shielding anisotropyparameters (span and skew) exhibit a wide rangeof values that exclusively depend on n values andtheir positional isomers (cis/trans or fac/mer) inSnClnBr6−n and SnBrnI6−n; this is responsible forbroader NMR linewidths (Figures 7, 8 and S20).For example, the computed 119Sn span is 1058ppm for trans-SnCl2Br4 in Cs2SnClxBr6−x and up to4702 ppm for trans-SnI4Br2 in Cs2SnBrxI6−x. Theexperimentally observed 119Sn NMR spectra inCs2SnClxBr6−x and Cs2SnBrxI6−x are a combinationof individual resonances of all possible SnClnBr6−n

or SnBrnI6−n species according to their populationsfollowing a random distribution (Table S12 andFigure S21). Along with magnetic shieldinganisotropy, the experimental NMR spectra getfurther broadened by both indirect and direct

spin-spin coupling between 119Sn and sixquadrupolar halogens (35/37Cl, 79/81Br and 127I) inSnClnBr6−n or SnBrnI6−n, vide supra. Due to thecomplex nature of these parameters, inability toacquire ultrafast MAS and less spectral resolutionamong ten possible unique halide arrangementsincluding the isomers in SnClnBr6−n or SnBrnI6−n, werefrain from quantitively deconvoluting theexperimental 119Sn NMR spectra for Cs2SnClxBr6−x

and Cs2SnBrxI6−x but these are discussedqualitatively below.

The multiple 119Sn peaks in mixed-halide samplescorrespond to several distinct local Sn octahedralenvironments. Guided by the DFT computations,the five spectrally distinguishable 119Sn NMRresonances in Cs2SnCl3Br3 are tentativelyassigned as the spectral regions for SnCl6 (−707ppm), SnCl5Br1/SnCl4Br2 (−966 ppm), SnCl3Br3

(−1291 ppm), SnCl2Br4/SnCl1Br5 (−1610 ppm) andSnBr6 (−1962 ppm) (Figure 7b). The 119Sn NMRspectra for Cs2SnBrxI6−x get much broader andunresolved due to the stronger 1J(119Sn−79/81Br)and 1J(119Sn−127I) in SnBrnI6−n along with theirhigher magnetic shielding anisotropy (1000sppm) predicted by DFT (Table S10). Nevertheless,four spectrally resolved 119Sn NMR resonances canbe tentatively assigned in Br-rich Cs2SnBr4.5I1.5 as

the spectral regions for SnBr6 (−1962 ppm),SnBr5I1 (−2434 ppm), SnBr4I2 (−2842 ppm) andSnBr3I3 (−3378 ppm) environments (Figure 7c).

Influence of Synthesis Routes on LocalStructure of Cs2SnI6. A feature of photovoltaicperovskites is that they can be synthesized byvarious avenues, with or without solvents;however, the local chemical structure resultingfrom these approaches is poorly understood.Thus, to elucidate the structure, solid-state 133Csand 119Sn NMR spectroscopy was performed forthree Cs2SnI6 samples prepared by three differentpreparation techniques, namely, solvent-free ball-milling, hand-grinding, and solvent-assistedsynthesis routes.

Figure 8. DFT computed results for SnClnBr6−n: (a)optimized geometries, (b) 119Sn isotropic chemicalshift as a function of number of Cl and (b) simulated119Sn NMR spectra.

The 133Cs NMR spectra of these three Cs2SnI6samples show resonances in the same spectralpositions with nearly identical linewidths (Figure9a). However, the slight asymmetry of theunderlying 133Cs resonance for the ball-milledsample corresponds to higher local structuraldisorder and smaller average crystallite sizes ascompared to solvent-synthesized and hand-ground samples. The 119Sn NMR spectra for allthree Cs2SnI6 samples show symmetricresonances with different chemical shifts andlinewidths (Figure 9b). The solvent-synthesizedand hand-ground samples exhibit sharperresonances at −4535 ppm (fwhm = 17 kHz) and−4527 ppm (fwhm = 17 kHz), respectively,whereas, the ball-milled sample shows a broadresonance at −4505 ppm (fwhm = 20 kHz). The

sharper 119Sn resonance indicates that thesolvent-synthesized sample has a well-orderedstructure with minimum defects, whereas, thebroader 119Sn resonance indicates the presence ofthe higher defect concentration in the ball-milledsample. The ball-milling is a high energy grindingtechnique that causes grinding-induced defectsas compared to the low energy manual hand-grinding route. The 119Sn spin-lattice relaxationdepends significantly on the synthesis routes withT1 = 0.01 s for the ball-milled sample, which is anorder of magnitude less than that of the solventsynthesized counterpart. The faster relaxationprocess for the ball-milled sample supports higherlocal structural disorder or defects which isconsistent with the conclusion based on 119SnNMR linewidths.

The higher structural disorder is also reflected inbroader peaks in powder XRD patterns for theball-milled sample as compared to the wellstructurally ordered solvent-synthesized sample(Figure S22). The different degrees of crystallinityin Cs2SnI6 samples also affects the opticalabsorption properties, with bandgaps of 1.56 eVfor the ball-milled sample and 1.42 eV for thesolvent-synthesized sample.

Figure 9. Solid-state (a) 133Cs (νr = 5 kHz, B0 =11.75 T) and (b) 119Sn (non-spinning, B0 = 7.05 T)NMR spectra of Cs2SnI6 samples prepared bysolvent-synthesis (SS), hand-grinding (HG), and ball-milling (BM) routes.

CONCLUSIONSThe macro to microscopic structures and bulkphotophysical properties for Cs2SnX6 (X = Cl, Br, I)parent perovskites prepared by the solvent-

assisted synthesis route were evaluated usingpowder XRD and multinuclear magneticresonance spectroscopy. Tin-119 NMRspectroscopy revealed a massive chemical shiftrange spanning ca. 4000 ppm that, according toDFT computations, is dominated by a large spin-orbit component. Likewise, the 119Sn spin-latticerelaxation times change by three orders ofmagnitude for these materials as the halide anionincreases in size. The 35Cl quadrupole couplingconstant for Cs2SnCl6 is nearly double that for thehybrid and non-hybrid lead(II) chlorideperovskites, APbCl3, consistent with Cl bound toSn in a 4+ oxidation state. The rapid octahedraldynamics of SnI6 units in Cs2SnI6 are remarkable,displaying a low activation energy (12.45 ± 0.87kJ/mol) for the polyhedral motion in these stable“static” solids.

The complete solid solutions Cs2SnClxBr6−x andCs2SnBrxI6−x (0 ≤ x ≤ 6) mixed-halides that exhibitattractive optical bandgaps spanningapproximately 3 eV, were successfully preparedby a solvent-free ball-milling synthesis route atroom temperature. According to solid-state 133Csand 119Sn NMR spectroscopy, the mixed-halideanalogues undergo a complete randomdistribution of Cl/Br or Br/I halogens at the atomiclevel, with no evidence of halogen-rich phasesegregation or domains under dark conditions.Furthermore, the broad 119Sn NMR resonancesobserved in mixed-halide samples, supported bymagnetic shielding parameters obtained by DFT,assisted in assigning the unique Sn chemicalenvironments, supporting the complex nature onemust contend with due to considerable shieldinganisotropy.

The bulk optical bandgap properties for theCs2SnI6 samples prepared by solvent-free andsolvent-assisted synthesis routes are sensitive tolocal structural changes as confirmed by NMR. Forexample, materials prepared by high energy ball-milling tend to show a lower degree ofcrystallinity (i.e., higher local structural disorder),resulting in faster 119Sn relaxation and a slightincrease in the bandgap (0.14 eV) compared tothe solvent-synthesized counterpart. Overall,solid-state NMR spectroscopy, alongside PXRDand DFT bestows unprecedented ability tounravel the complex chemical structure anddynamics present within vacancy-ordered doubleperovskite materials, whereby random halidearrangements and defects govern the opticalbandgaps.

ASSOCIATED CONTENT Supporting Information. Detailed discussion of synthesis, experimentaltechniques, DFT computations, Tables S1-S12 andFigures S1-S22 are available in the SupportingInformation (PDF).

AUTHOR INFORMATION

Corresponding Author*e-mail: vladimir.michaelis@ualberta.ca

Author ContributionsAll authors have given approval to the final version of the manuscript. NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge the Natural Sciences andEngineering Research Council (NSERC) of Canada,Canada Foundation for Innovation, Government ofAlberta, Future Energy Systems (CFREF), CREATE(ATUMS) and the University of Alberta for researchsupport. AK acknowledges an Alberta InnovatesGraduate Scholarship. VAMG is supported by aConsejo Nacional de Ciencia y Tecnologia(CONACyT), scholarship No. 291163. A part of theresearch was carried out on computational resourcesprovided by Compute Canada/ Calcul Canada(www.computecanada.ca).

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