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Carbon clathrate materials, comprised of sp3 carbon structures with light elements, as well as polymeric carbon monoxide, represent two classes of high-pressure materials with promising potential structural and energetic applications. A systematic exploration of PTx space was performed in carbon+ alkali and alkaline earth metals, under high-pressure/temperature conditions, as well as silicon-based systems, to establish the propensity to for sp3-based carbon/silicon networks with superlative properties. For the case of silicon, it was unambiguously determined that silicon clathrates are thermodynamically stable at high-pressure conditions. This suggests that other high-

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Pressure-induced Formation of Energetic and Structural Extended Solids with Quench-recovery to Ambient Conditions

The views, opinions and/or findings contained in this report are those of the author(s) and should not contrued as an official Department of the Army position, policy or decision, unless so designated by other documentation.

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U.S. Army Research Office P.O. Box 12211 Research Triangle Park, NC 27709-2211

high pressure, synthesis, clathrate, poly-CO

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19a. NAME OF RESPONSIBLE PERSON

19b. TELEPHONE NUMBERTimothy Strobel

T.A. Strobel, M. Somayazulu, R.J. Hemley, O.O. Kurakevych

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Carnegie Institution of Washington1530 P Street NW

Washington, DC 20005 -1910

ABSTRACT

Pressure-induced Formation of Energetic and Structural Extended Solids with Quench-recovery to Ambient Conditions

Report Title

Carbon clathrate materials, comprised of sp3 carbon structures with light elements, as well as polymeric carbon monoxide, represent two classes of high-pressure materials with promising potential structural and energetic applications. A systematic exploration of PTx space was performed in carbon+ alkali and alkaline earth metals, under high-pressure/temperature conditions, as well as silicon-based systems, to establish the propensity to for sp3-based carbon/silicon networks with superlative properties. For the case of silicon, it was unambiguously determined that silicon clathrates are thermodynamically stable at high-pressure conditions. This suggests that other high-pressure phases may be synthesized outside their thermodynamic stability regime and elucidates the first example of the chemical synthesis of a high-pressure phase at ambient-pressure conditions. We examined the sodium removal from a newly discovered NaSi6 (Na4Si24) compound. Upon complete sodium removal, a new sp3 silicon allotrope was produced that possesses a quasidirect band gap with the ideal value for photovoltaic applications. For the case of carbon, mixtures with Li, Mg, Na, and Ca were investigated. All systems showed known carbide formation under certain conditions, but several new and recoverable phases were identified including Mg2C, Mg2C3, Ca2C, Ca2C3 and Ca3C2. Some of these, such as Mg2C, contain unusual anions like C4- that are recoverable to ambient conditions and display sp3 carbon hybridization. These may serve as useful precursors for carbon clathrate materials. The polymerization behavior of CO (poly-CO, p-CO) was examined over a range of conditions with different dopants. The presence of molecular nitrogen and acetylene did not significantly affect the polymerization pressure of CO or the molecular structure of the resulting polymer. The presence of amorphous carbon and SiO2 appear to reduce the polymerization pressure by a fraction of one GPa. The presence of metallic lithium and sodium appear to reduce the polymerization pressure substantially. These additives may be considered as a route to producing larger quantities of p-CO.

(a) Papers published in peer-reviewed journals (N/A for none)

Enter List of papers submitted or published that acknowledge ARO support from the start of the project to the date of this printing. List the papers, including journal references, in the following categories:

06/12/2014

08/29/2013

08/29/2013

Received Paper

6.00

2.00

3.00

Oleksandr O. Kurakevych, Yann Le Godec, Timothy A. Strobel, Duck Young Kim, Wilson A. Crichton, Jérémy Guignard. High-Pressure and High-Temperature Stability of Antifluorite Mg, The Journal of Physical Chemistry C, (04 2014): 0. doi: 10.1021/jp5010314

Oleksandr O. Kurakevych, Timothy A. Strobel, Duck Young Kim, Takaki Muramatsu, Viktor V. Struzhkin. Na-Si Clathrates Are High-Pressure Phases: A Melt-Based Route to Control Stoichiometry and Properties, Crystal Growth & Design, (01 2013): 0. doi: 10.1021/cg3017084

George D. Cody, Oleksandr O. Kurakevych, Timothy A. Strobel, Duck Young Kim. Synthesis of Mg2C: A Magnesium Methanide, Angewandte Chemie International Edition, (08 2013): 0. doi: 10.1002/anie.201303463

TOTAL: 3

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Peer-Reviewed Conference Proceeding publications (other than abstracts):

6.00

(b) Papers published in non-peer-reviewed journals (N/A for none)

1) High Pressure Gordon Research Conference, Biddeford, ME, June 24-29, 2012 2) Atomic and Intermolecular Interactions Gordon Research Conference, Easton, MA July 15-20, 2012 3) Strobel, T.A., Neighborhood Public Lecture, Carnegie Institution of Washington, Washington, DC (2012). 4) Strobel, T.A., Chemical Engineering Departmental Seminar, Colorado School of Mines, Golden, CO (2013). 5) Strobel, T.A., Chemistry Departmental Seminar, Naval Research Laboratory, Washington, DC (2013). 6)Kurakevych, O.O.; Le Godec, Y.; Strobel, T.A.; Turkevich, V.Z.; Solozhenoko, V.L., IUCrWorkshop: Advances in Static and Dynamic High-Pressure Crystallography, Hamburg, Germany (2013).

(c) Presentations

Number of Presentations:

Non Peer-Reviewed Conference Proceeding publications (other than abstracts):

Received Paper

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(d) Manuscripts

06/12/2014

08/29/2013

08/30/2012

Received Paper

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Timothy A. Strobel,†,*, Oleksandr O. Kurakevych,†,‡ , Duck Young Kim,† , Yann Le Godec,‡ , Wilson Crichton,§, Jérémy Guignard,§ , Nicolas Guignot,? , George D. Cody† , Artem R. Oganov. Synthesis of ??Mg2C3: A Monoclinic High-Pressure Polymorph of2 Magnesium Sesquicarbide, Inorganic Chemistry (02 2014)

DuckYoung Kim, Stevce Stefanoski, Oleksandr O. Kurakevych, Timothy A. Strobel. A new allotrope of silicon with a quasidirect band gap, To be submitted for publication (08 2013)

Oleksandr O. Kurakevych, Timothy A. Strobel, Duck Young Kim, Takaki Muramatsu, Viktor V. Struzhkin. Na-Si Clathrates are high-pressure phases: A melt-based route to control stoichiometry and properties, SUBMITTED (07 2012)

TOTAL: 3

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Awards

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Names of Post Doctorates

Names of Faculty Supported

Names of Under Graduate students supported

U.S. Provisional App. No. 61/843,581, filed July 8, 2013, and U.S. Provisional App. No. 61/874,582, filed September 6, 2013.

(1) Invited lecture at Atomic and Intermolecular Interactions Gordon Research Conference, Easton, MA July 15-20, 2012. (2) Mg2C publication (Angewandte Chemie) featured on inside back cover

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Scientific Progress

We have investigated the high-pressure / high-temperature (HP / HT) synthesis of novel structural and energetic solids, based on carbon and oxygen-carbon frameworks, which are expected to exhibit exceptional materials properties, show ambient quench recoverability and have potential for larger-volume scaling. We have examined a broad range of PTx conditions in the carbon (silicon) + alkali or alkaline earth metal systems to determine the extent of HP compound formation, ambient recoverability and new materials properties. We have investigated the polymerization of carbon monoxide (p-CO) under HP conditions in the presence of dopants and catalytic additives to determine effect on polymerization onset pressure and chemical stability. Summary of the most important results are enumerated below as concise conclusions, supporting discussion with tables & figures are provided in the attached document. 1)Sodium-silicon clathrates are HP phases. This indicates that HP phases may be synthesized through chemical precursors at ambient conditions. 2)A new structure, NaSi6, was discovered at HP. This phase is recoverable to ambient conditions and shows unusual, potentially quasi-one-dimensional, electrical resistivity. 3)By treating a NaSi6 precursor compound (Na4Si24 unit cell) we were able to produce a new pure allotrope of silicon (Si24) that has a quasidirect band gap. 4)Carbon + light metal systems show rich behavior under HP/HT conditions. Several previously unreported structures and compounds were discovered. 5)A new carbide, Mg2C, was discovered at HP/HT conditions. This phase is fully recoverable to ambient conditions. It has a bulk modulus of 87 GPa and is electrically insulating. This compound could potentially act as a solid-based fuel source as it is readily hydrolyzed to form CH4 gas. 6)The thermodynamic properties of Mg2C were established and the conditions of high-pressure, high-temperature stability have been determined. 7)A new polymorph of Mg2C3 carbide was discovered and the crystal structure was solved using a combination of experimental and theoretical methods. 8)New structures in the Ca+C system were identified. Some of these may contain unique polymeric carbon units. 9)Methods were developed to prepare samples of p-CO. This can be done routinely for samples with dimensions of 1000 micron diameter x 150 micron thickness. 10)The polymerization pressure of CO in the presence of N2 and C2H4 is not reduced from the pure CO sample conditions. These dopants tend to phase separate with no significant alteration of the chemical structure; however, subtle vdW-type interaction was observed. 11)Neither seed crystals of p-CO nor nano-sized Pd powder reduce the pressure required for CO polymerization at ambient temperature. 12)Amorphous carbon appears to reduce the polymerization onset pressure of CO by ~0.5 GPa when compared with the pure system. It is unclear whether this is a catalytic (nucleation) or thermodynamic (chemical stabilization) effect. 13)The polymerization pressure of CO was investigated in the presence of several different metal and non-metal catalysts. Metallic lithium and sodium, as well as non-metallic amorphous carbon and SiO2, appear to reduce the pressure needed for CO polymerization.

Technology Transfer

1  

FinalReportAttachment:DiscussionandfiguresProposalNumber:60587EGDRPAgreementNumber:W911NF1110300ProposalTitle:Pressure‐inducedFormationofEnergeticandStructuralExtendedSolidswithQuench‐recoverytoAmbientConditionsReportPeriodBeginDate:09/01/2012ReportPeriodEndDate:02/28/2014T.A.Strobel,M.Somayazulu,R.J.Hemley,O.O.KurakevychAbstractCarbon clathrate materials, comprised of sp3 carbon structures with light elements, as well aspolymeric carbon monoxide, represent two classes of high‐pressure materials with promisingpotential structural and energetic applications. A systematic exploration of PTx space wasperformed in carbon+ alkali and alkaline earth metals, under high‐pressure/temperatureconditions, as well as silicon‐based systems, to establish the propensity to for sp3‐basedcarbon/siliconnetworkswithsuperlativeproperties.Forthecaseofsilicon,itwasunambiguouslydeterminedthatsiliconclathratesarethermodynamicallystableathigh‐pressureconditions.Thissuggeststhatotherhigh‐pressurephasesmaybesynthesizedoutsidetheirthermodynamicstabilityregime and elucidates the first example of the chemical synthesis of a high‐pressure phase atambient‐pressure conditions. We examined the sodium removal from a newly discoveredNaSi6(Na4Si24)compound.Uponcompletesodiumremoval,anewsp3siliconallotropewasproducedthatpossessesaquasidirectbandgapwiththeidealvalueforphotovoltaicapplications.Forthecaseofcarbon,mixtureswith Li, Mg, Na, and Cawere investigated. All systems showed known carbideformation under certain conditions, but several new and recoverable phases were identifiedincludingMg2C,Mg2C3,Ca2C,Ca2C3andCa3C2.Someofthese,suchasMg2C,containunusualanionslikeC4‐thatarerecoverabletoambientconditionsanddisplaysp3carbonhybridization.Thesemayserveasusefulprecursorsforcarbonclathratematerials.ThepolymerizationbehaviorofCO(poly‐CO, p‐CO) was examined over a range of conditions with different dopants. The presence ofmolecularnitrogenandacetylenedidnotsignificantlyaffectthepolymerizationpressureofCOorthe molecular structure of the resulting polymer. The presence of amorphous carbon and SiO2appear to reduce thepolymerizationpressureby a fractionofoneGPa.Thepresenceofmetalliclithium and sodium appear to reduce the polymerization pressure substantially. These additivesmaybeconsideredasaroutetoproducinglargerquantitiesofp‐CO.Scientificprogressandaccomplishments

1) Sodium‐siliconclathratesareHPphases.ThisindicatesthatHPphasesmaybesynthesizedthroughchemicalprecursorsatambientconditions.

For the first time we have demonstrated unambiguously that Na+Si clathrates arethermodynamically stable high‐pressure phases. Sodium clathrates were prepared fromelementalNa/Simixtures (20at%ofNa, i.e.,~5%excessas comparedwithstoichiometric sIand sII clathrates) using the multi‐anvil press technique at pressures between 1‐8 GPa andtemperaturesbetween700‐1275K.Figure1showsthex‐raydiffractionpatternobtainedfromasamplethatwasrecoveredfrom6GPaand1100K.Thex‐raydiffractionpatternindicatesthat

2  

sIclathratemaybesynthesizeddirectlyfromtheelementsunderHP/HTconditions.InadditionwehaveperformedDFTcalculationsthatconfirmthehigh‐pressurethermodynamicstability.

Figure1.(a)X‐raydiffractionpatternofsIclathratesynthesizedat6GPaand1100K.(b)ImagesofmetallicsI(Na8Si46),scalebaris50microns.(c)atomicstructureofsIclathrate.Theseresultsopennewperspectivesforhigh‐pressuresynthesisandpropertiescontrolofnewadvancedmaterials.Thehigh‐pressurethermodynamicstabilityofNa‐Siclathratephasesallowsfor amelt‐based synthesis approach,which couldbe veryuseful for compositional control inmixed phases (e.g., Na+Ba etc.), high‐quality single crystals, and for precise tuning of theoccupancyratios.Allphasesformedinthispressuredomainallowforlarger‐volumescalingofmaterials (from 40 cm3 for cubic sI at ~3 GPa to 1 cm3 at ~8 GPa). Our results reveal theexistence of multiple chemical mechanisms that allow for synthesis of high‐pressure phases“withoutpressure”.Because the thermodynamic stability fieldsofNa‐Si clathratephasesonlyexist under high‐pressure conditions, previous reports of these structuresmay be viewed asnon‐equilibrium, precursor‐based syntheses of high‐pressure phases at low‐pressureconditions. The understanding of such intrinsic interrelationships between thermodynamicsandkineticsisthusthenextsteptoexplorethatcouldopenpotentialforotherprecursor‐basedsynthesis of other high‐pressure phases, especially carbon‐based materials. Na‐Si clathrateformationmay be viewed as the first chemical example of a high‐pressure phase at ambientconditions.

2) Anewstructure,NaSi6,wasdiscoveredatHP.Thisphaseisrecoverabletoambientconditionsandshowsunusual,potentiallyquasi‐one‐dimensional,electricalresistivity.

DuringthesynthesisofNa+Siclathratephases,theformationofanovelNa‐Sicompound,NaSi6was observed when pressure was increased to 8GPa. It has the Eu4Ga8Ge16 structural type(Fig.2),neverreportedthusfarforanalkalimetal.Thestructureiscomposedofsp3‐bondedSiatoms,whichformtunnels,intercalatingNaatomsalongthea‐axis.AscomparedtoBaSi6,SrSi6,andCaSi6,thecorrespondingsodiumcompoundformsatsubstantiallylowerpressure:8GPaascompared to 11.5GPa and 10 GPa for BaSi6 and CaSi6, respectively. The pressure for NaSi6formationallowsconsiderationofthisphaseforalarge‐volumeproduction,e.g., inthetoroid‐typehigh‐pressuresystems,contrarytosimilarcompoundsofalkali‐earthmetals.

3  

Figure2.(a)X‐raydiffractionpatternofNaSi6at8GPaand1100K.(b)ImagesofmetallicNaSi6,scalebaris50microns.(c)AtomicstructureofNaSi6.Theelectrical resistivityofNaSi6 asa functionof temperaturewasmeasuredby the standardfour‐electrode technique, using a Physical Property Measurement System (PPMS: QuantumDesign, Inc.).Overall,NaSi6exhibitsmetallicbehavior,with theelectricalresistivity increasingapproximately as a parabolic curve with temperature above 75 K (Fig. 3). Below 75K, theelectrical resistivity begins to increase with further temperature decrease, a feature neverobservedsofarforrelatedcompoundswithinthisstructuralfamily.Theobservedminimumatabout75Kmightbecausedbythecomplexityoftheelectronicbandstructure,particularlytheone‐dimensionalnatureoftheNachannels,oranunknownmagneticscatteringprocess.

Figure3.ElectricalresistivityofNaSi6asafunctionoftemperature.Insetshowsthecalculatedbandstructureat1MPa.

3) BytreatingaNaSi6precursorcompound(Na4Si24unitcell)wewereabletoproduceanewpureallotropeofsilicon(Si24)thathasaquasidirectbandgap.

Previously, we demonstrated unambiguously that Na+Si clathrates are thermodynamicallystablehigh‐pressurephases,andalsoreportedthediscoveryofanewNaSi6(Na4Si24unitcell)compound. Thiscompoundconsistsofachannel‐likesp3siliconhoststructurewithchainsof

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sodium atoms as a guest structure. These open channels that host sodium atoms suggest apossiblepathwayforsodiumremovalviadiffusionalongthechannelsasschematicallyshowninFigure4.

Figure4.(a)SchematicofcompositionalchangesfromNa4Si24(left)toSi24(right).Naatomsareshown in purple and silicon in yellow. (b) Si24 unit cell exhibiting three crystallographicallyunique positions in each color (c) Fractional view of Si24 emphasizing its channel structure.Channelsareformedbyeight‐memberringsalongthea‐axis,whicharelinkedbysix‐memberedrings on the top and sides. These channels are connected along the c‐axis by five‐memberedrings.ByexposingtherecoveredNa4Si24samplestoelevatedtemperatures,removalofNaatomsfromthestructurewasobserved.Thisprocessoccursattemperaturesaslowas320K,whiletype‐IIsilicon clathrates (NaxSi136) require much higher temperatures (> 623 K) for Na removal.Thermal“degassing”ofNa4Si24at400Kunderdynamicvacuumconditionsresultedinagradualreduction of the sodium content and sodiumwas completely removed from structure over aperiod of eight days. The structure of the resulting Na‐free structure is unchanged (Cmcm),however, the lattice constants are slightly modified.We verified the absence of Na from thestructureusingpowderX‐raydiffractionandenergydispersiveX‐rayspectroscopy.Nosodiumwasfoundwithinour instrumentaldetection limits(0.1atom%), therefore, thenewstructuremaybeconsideredasanewallotropeofsilicon.WeinvestigatedtheelectronicandopticalpropertiesoftheSi24allotropeusingacombinationofexperimental and theoretical methods (Figure 5). The new structure possess a quasidirectbandgap of ~1.3 eV, which suggests that this structure will be a very useful optoelectronicmaterial. The quasidirect band structure of Si24 overcomes one of the most fundamentallimitationsofthe“normal”diamond‐structuredsiliconphase,whichpossessesanindirectbandgap.

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Figure5.ElectronicandopticalpropertiesofSi24.(a)CalculatedSi24bandstructure(PBE).(b)Zoomed in region of band gap with table showing DFT (PBE) and G0W0 corrected values.Arrowsindicatedirect(Ed)andindirect(Ei)gaps.(c)ElectricalconductivityofNa4Si24andSi24(inset shows fit of intrinsic conductivity region with band gap of 1.3 eV). (d) Tauc plots ofKubelka‐Munk absorption (K/S) for Si24 obtained from optical reflectivity measurements.Absorptionedgesareobservedat1.29eVand1.39eVassumingindirectanddirecttransitions,respectively

4) Carbon+lightmetalsystemsshowrichbehaviorunderHP/HTconditions.Several

previouslyunreportedstructuresandcompoundswerediscovered.

The phase behavior and structural evolution of several carbon + light metal systems wereinvestigated under HP/HT conditions. Sources of carbon have included glassy amorphouscarbon,nanocarbonandgraphite,whilelightsmetalshavebeenlimitedtoLi,Mg,Na,andCa.Resultsaresummarizedasfollows:

Below5GPatheformationofLi2C2(knowncarbidephase)wasobserved.Above12GPa,at least one new phase was discovered in the Li+C system. Figure 6 shows x‐raydiffractionandRamanspectraobtainedfromthisphase,whichcannotbedescribedbyanyknowLi+Ccompounds.Ramanmodesobservednear1200cm‐1maybeindicativeofsp3‐type carbon bonding. All of the phases determined thus far are recoverable toambientconditions.

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Figure 6. (a) X‐ray diffraction pattern showing new peaks for Li+C structure. (b) RamanspectrumLi+Cphasesynthesizedat12GPa.

Upto20GPa,wehaveonlyobservedtheformationoftheknowncarbideNa2C2inthe

Na+Csystem.

Below5GPatheformationofCaC2(knowncarbidephase)wasobserved.Above7GPaseveralnewphaseswerediscovered(seebelow).

The Mg+C system was studied extensively – two new recoverable HP phases were

discovered(seebelow).

5) Anewcarbide,Mg2C,wasdiscoveredatHP/HTconditions.Thisphaseisfullyrecoverable

toambientconditions.Ithasabulkmodulusof87GPaandiselectricallyinsulating.Thiscompoundcouldpotentiallyactasasolid‐basedfuelsourceasitisreadilyhydrolyzedtoformCH4gas.TheMg+Ccarbonwasinvestigatedoverabroadrangeofsynthesisconditions:7‐70mol%Mg,1‐30 GPa and 500‐2000 K. Various sources of carbon have been tested, including glassyamorphouscarbon,nanocarbonandgraphite.At thecomposition66mol%Mgandpressureabove15GPa,theformationofnearlyphase‐pureMg2Cwasdiscovered.Figure6showsanxrdpattern obtained from a synthesis run recovered from 15 GPa and ~1700 K and thecorresponding Mg2C structure. Confirmation of the [C4‐] methanide ion was performed byhydrolyzingsamples,whichproducedCH4gas.Figure7showsthecrystalstructureofMg2Candstructuralcoordination.CarbonwithinMg2Cisis 8‐fold coordinatedbymagnesium,whereas carbon coordinationwithinMg2C3 andMgC2 ismuchmoresophisticated. Ifoneconsidersthewholecarbonanionsasstructuralunits,Mg2C3andMg2Chavethesamecoordinationnumber8,butinthefirstcasetheyformadistortedandelongated dodecahedron, while in the second case the coordination polyhedron is a regular

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cube. InMgC2 theC2dumbbell coordinationnumber is6 (elongatedoctahedron).Contrary toMg2C3 andMgC2, Mg2C does not contain covalent C‐C bonds. Rather, carbon exists in a veryunusualC4‐ionicstate.The 13C NMR spectrum (Fig. 7c) of Mg213C sample (99% of isotopic purity) displays oneresonanceatδ+102ppm.Thisagreeswith theantifluorite structurewhichcontainsonlyoneuniquecrystallographicpositionforcarbon.ThechemicalshiftissignificantlyhigherthanthatofC4‐inAl4C3(34and51ppmforCatomsintwocrystallographicallydifferentsites),andagreeswell with higher degree of covalent character of Al‐C bonding as compared with Mg‐C. TheabsenceofJ‐couplingintheobservedNMRspectrumsuggeststhatcarbonwithinthecompoundisprimarilyionic;increasedcovalencywouldbemanifestedbythepresenceofJ‐coupling.

Figure 7. (a) X‐ray diffraction data with Mo K radiation (points), Rietveld refinement (line),difference(lower line).TickmarksareshownforMg2C(top)andMgOimpurity(bottom).(b)Carbon andmagnesium coordination inMg2C. (c) NMR spectrum of Mg213C (99% of isotopepurity).Togainfurther insights intotheionicchemicalbonding,wecalculatedtheelectrondensity inthestructureandperformedaBader‐typebondinganalysis(Fig.8).Theseresultsconfirmthehighly ionic nature of the solid: the effective charges on Mg and C are +1.57 and ‐3.14,respectively. To our knowledge, this is the most negative effective charge of carbon everachieved,while the C4‐ ionic radius inMg2C has the highest value of 1.50Å. Themagnesiumcharge is the lowest among other known carbides (+1.65 and +1.7 for Mg2C3 and MgC2,respectively). The charge density in the (110) plane shows a nearly spherical distribution ofelectronscenteredatMgandCionswithalowelectrondensityintheinterstitialregions(Fig.8a). The electron localization function (ELF, Fig. 8b) provides a closer look at the bondingnature.ELFminimaobservedbetweenMgandCatomsconfirmtheclosed‐shellionicnatureofthebonding.Thereisno"shared‐electron"picture,whichischaracteristicofcovalentbondingandrequiresELFmaximabetweenatoms.

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Figure8.(a)NormalizedelectrondensitydistributioninMg2C,coloredbluetored(scale0to1).(b)Electronlocalizationfunction.Delocalizedtolocalizedelectrons(ELF)coloredfromblueto red (scale from 0 to 1). Small and large spheres represent carbon and magnesium,respectively.TheC4‐characterofthiscompoundisveryinteresting.Althoughionic,thiscarbonexistsinansp3hybridizedstateandisstableatambientconditions.WespeculatethatMg2Cwillserveasavery valuable precursor for the formation of diamond‐like sp3 carbon at reduced pressureconditions. A direct analogy may be drawn between Mg2C and Zintl precursors like Na4Si4,which are known to form sp3 clathrate structures at ambient pressure upon thermaldecomposition(despite the fact that theclathratesareonly thermodynamicallystableathighpressure).

6) ThethermodynamicpropertiesofMg2Cwereestablishedandtheconditionsofhigh‐pressure,high‐temperaturestabilityhavebeendetermined.Thehigh‐pressureandhigh‐temperaturestabilityofantifluoriteMg2CwasstudiedbyusinginsituX‐raydiffractionupto20GPaand1550K.Acombinationofabinitiocalculationsofrelativeenergies and phonon density of states as a function of pressure insights to the formation ofMg2C under extreme conditions and ability to recoverability it at ambient conditions. OurresultsindicatethathighpressureisacrucialpointforthethermodynamicstabilizationofC4‐anion in the antifluorite structure, although this anion is fully recoverable to ambientconditions.Figure9showsasequenceofinsituX‐raypowderdiffractionpatternsofMg‐Csample,takenduringcompressionto18GPa(at300K),withheatingto1550K(at~18GPa)andsubsequentisothermaldecompression(at~1550K).TheformationofMg2Cwasobservedduringheatingat18GPaafterthenon‐equilibriummeltingofMgat~1500K.At1550KMg2Cdecomposesatthepressureof~12GPa.Aconstantcompositionsection(33at.%C)ofthep‐TphasediagramofMg2Cwasestablishedusingourinsitudataat1550Kandthe0Kvaluepredictedbyabinitiocalculations(stabilitycomparedtoMg‐diamondmixture).ThemeltingtemperaturesofMg2Cat1 MPa and 15 GPa were then estimated using a Lindemann mode combined with ab initiocalculationsoftheDebyetemperatureunderpressureandexperimentalp‐Vequationofstate.

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Figure9.ExperimentalX‐raydiffractiondataobtainedinsituduringtheHPHTformationofMg2C(left).Structuresofgraphite,Mg2C,anddiamond(center),PhasediagramdeterminedforMg2C(right).7) AnewpolymorphofMg2C3carbidewasdiscoveredandthecrystalstructurewassolved

usingacombinationofexperimentalandtheoreticalmethods.High‐pressure,high‐temperaturestudieswereconductedintheMg+CsystematcompositionsotherthanMg2C,namely,Mg2C3.Inthiscase,anotherstructurewasidentifiedwithamuchmorecomplex structure than cubic Mg2C. With the assistance of ab initio structure searchingmethods,wewereabletosuccessfullysolvethestructure(Figure10) from insituX‐raydata.LikethepreviouslyknownorthorhombicPnnmstructure(‐Mg2C3),thenewmonoclinicC2/mstructure(‐Mg2C3)containslinearC34‐chainsthatareisoelectronicwithCO2.Unlike‐Mg2C3,whichcontainsalternating layersofC34‐ chainsoriented inoppositedirections,allC34‐ chainswithin‐Mg2C3arenearlyalignedalongthecrystallographicc‐axis.Hydrolysisof‐Mg2C3yieldsC3H4,asdetectedbymassspectrometry,whileRamanandNMRmeasurementsshowclearC=Cstretching near 1200 cm‐1 and 13C resonances confirming the presence of the rare allylenideanion.

Figure10.Insituangulardispersivediffractionpatternfor‐Mg2C3synthesizedinthePEpressandrecoveredtoambientpressure.Rietveldrefinementwasemployed forthe‐Mg2C3struc‐ture, while Le Bail analysis was used for MgO and Mg due to strong preferred orientation

6 7 8 9 10 11 12 13 142° ( = 0.3188 Å)

c)

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causedbyrecrystallizationandcapsuletexture.Tickmarksindicate‐Mg2C3,MgandMgOfromtoptobottom.ThenewMg2C3structureismonoclinicwithspacegroupC2/mwitha=4.8,b=4.7,c=6.0Åandb=126°.Thisphaseformsabove~6GPaandisstableto~15GPa.Observationssuggestthat this structure transforms to Mg2C at higher pressure (with heating and depending onglobalcomposition).UnlikeMg2C,Mg2C3containslinearC34‐chainsthatarechargebalancedbyMg2+cations(Fig.11).

Figure 11. C34‐ coordination in and Mg2C3 and histogram of nearest neighbor Mg‐Cdistancesoutto3A.

8) Newstructures in theCa+Csystemwere identified.Someof thesemaycontainunique

polymericcarbonunits.Studieswere performed in the Ca+C system to investigate the influence of Ca on sp3 carbonnetworkformation.Atpressures<4GPa,weobservedtheformationofCaC2calciumcarbide,aknownstructurewithlinearC22‐dumbbells.Athigherpressure,newstructureswereidentified.Figure 12 shows in situ X‐ray diffraction patterns obtained at 20 GPa and 600‐900 K. ThestartingFCC(orBCCdependingonpressure)metalclearlytransformsintoa lowersymmetrystructure(s).

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Figure12.X‐raydiffractionpatternsobtainedforCa+Cmixturesat20GPaheatedto600K

(bottom)and900K(top).Lowsymmetrystructuresareclearlyidentified.With theaidof ab initio structural searching,wehave identified candidate structures for thenew Ca+C compounds. Figure 13 shows possible structure and corresponding XRD patternsthat match closest to the experimental data. Both of these structures (P‐1 and Immmsymmetries)contain1Dcarbonnanochains.

Figure13.SimulatedX‐raydiffractionpatternsandcrystalstructuresfornewCaC2compounds

withP‐1symmetry(top)andImmmsymmetry(bottom).

These structures are very unique in the sense that they contain polymerized carbon nano‐ribbons.Thesephases shouldbemetallicwithvery interestingproperties.Furthermore, theyoffer the possibility of extracting 1D carbon nanochains, which may have very interestingmechanicalproperties.

5 7 9 11 13 15

2 (degrees)

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In addition to this, we have identified three new carbide structures up to 30 GPa, using acombination of X‐ray diffractionmethods and ab initio crystal structure searching (Fig. 14).Ca2CcontainsC4‐ anions, similar toMg2C,however, thestructure isorthorhombic.MonoclinicCa2C3isisostructuralwith‐Mg2C3,butnoamodificationisknownforthecaseofCa.Finally,anentirelynewcomposition,Ca3C2,wasdiscovered.Thisstructure ismetallicanddoesnothaveformalchargebalance.

Figure14.StructuresandX‐raydiffractionpatternsfornewCa‐Ccarbides.

9) Methodswere developed to prepare samples of p‐CO. This can be done routinely forsampleswithdimensionsof1000microndiameterx150micronthickness.Wehavedevelopedexperimentalproceduresto loadCOintodiamondanvilcells(DAC)usingbothcryogenicandhigh‐pressuregasmethods.Thesetechniquesareextremelyreliable(near100% loading efficiency). When utilizing the supported anvil design (Boehler‐Almax style),largesamplesmaybeproduced(1000microndiameterx150micronthicknessat7GPa).

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10) ThepolymerizationpressureofCOinthepresenceofN2andC2H4isnotreducedfromthe

pure CO sample conditions.Thesedopants tend tophase separatewithno significantalterationofthechemicalstructure;however,subtlevdW‐typeinteractionwasobserved.ExperimentswereconductedonmixturesofCOandN2orC2H4todetermineifpolymerizationin the presence of these compounds would decrease the polymerization onset pressure orincrease the chemical stability through bonding with the p‐CO network. Neither of thecomponentswasfoundtoreducethepolymerizationpressurerequiredforpureCO(~6GPaatroomtemperature).BothN2andC2H4exhibitedphaseseparationfromp‐COandinfrared(IR)spectraobtainedfromthedifferentsamplesdidnotindicateanysignificantbondingchangesinthepolymer structure.However, subtle shifts inRamanmodes suggest thepresenceofweakvan derWaals interaction. Figure 15 shows images of p‐COwithN2 and C2H4, aswell as thecorrespondingIRspectra.

Figure15.(a)ImageofN2+p‐COmixtureand(b)C2H4+p‐COmixtureshowingphaseseparationofthecomponents.(c)IRspectraforthedifferentsamplesindicatingnosignificantchangesinchemicalbonding.ForthecaseofC2H4,newIRmodesappearwhichoriginatefrompureC2H4.

11) Neitherseedcrystalsofp‐COnornano‐sizedPdpowderreducethepressurerequiredforCOpolymerizationatambienttemperature.In order to reduce the polymerization inset pressure for pure CO, attempts were made tocatalytically induce (reduce nucleation barrier) p‐CO formation using nano‐Pd powder and aseedcrystalofpurep‐CO. Neitherof theseadditiveswasfoundtoreducethepolymerizationonset pressure. Figure 16 shows the seed crystal of p‐CO surrounded by molecular CO andcorrespondingIRspectra.

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Figure16.Imageofp‐COseedcrystalinsidediamondanvilcellat5.9GPasurroundedby

molecularCO.ThemolecularCOdoesnotpolymerize,whileincontactwiththeseed.IRspectraareshownatbottom.

12) Amorphouscarbonappears toreduce thepolymerizationonsetpressureofCOby~0.5GPa when compared with the pure system. It is unclear whether this is a catalytic(nucleation)orthermodynamic(chemicalstabilization)effect.The polymerization of CO was investigated in the presence of amorphous nano‐carbon.ComparedwithpureCO,whichalwaysshowsapolymerizationonsetpressurebetween5.5‐7GPadependingontheexactexperimentalconditions, thesample incontactwithnanocarbonstartedtopolymerizeatonly4.8GPa.Thisrepresentsanapproximate0.5GPadecreaseinthepressurerequiredforCOpolymerizationinthepuresystem.Figure17showsanimageoftheCO incontactwithnanocarbonandthecorresponding IRspectrawithpressure. It isunclearwhether thisprocess iscatalytic innatureor if thecarbonplaysachemicallystabilizingrole.Thedetaileddeterminationofchemicalstabilityiscurrentlyunderinvestigation.

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Figure17.ImageofmolecularCOincontactwithnanocarbonbeforepolymerizationandIRspectraobtainedwithincreasingpressureindicatingpolymerizationonsetat4.8GPa.

13) ThepolymerizationpressureofCOwasinvestigatedinthepresenceofseveraldifferentmetal and non‐metal catalysts. Metallic lithium and sodium, aswell as non‐metallicamorphous carbon and SiO2, appear to reduce the pressure needed for COpolymerization.WehaveusedacombinationofIRandRamanspectroscopystudiestoexplorethepossibilityoflowering the polymerization pressure of CO. We undertook three separate routes tosystematically explore the lowering of polymerization pressure in CO. In the first study, weattempted to use the fact that several groups that have used diamond anvil cells reported arangeofpressuresdependingonthegasketmaterial.Wehypothesizedthatthiscouldindicatemetal‐CO interaction and therefore a dependence of this interaction on the polymerizationpressure.We chose to standardize the protocol by using Au lined stainless steel gaskets and IRspectrometryusing thepressureshiftof theantisymmetricNO2stretchasapressuresensorandtheappearanceofcarbonylbands(at720cm‐1)andtheepoxybands(at830cm‐1).Thiswasnecessarytoreducetheeffectofphoto‐reactivityandalsoestablishtheformationofthecorrectpolymeric phase. To strengthen the signal from the sample and proper referencing,we usedgasketswithdualholes,one filledwith theKNO2‐KBrmixtureandtheothercompletely filledwithCO. The gasket geometrywas first calibratedusing ruby andN2 in the sample chamberinsteadofCO.Once this protocolwas established,we used severalmetal additives such as nano‐Pd, Pt, Pt‐black,Al,Na,LiaswellascommongasketmaterialssuchasRe,W,FeandCu.Wealsotestednon‐metallicadditivesofamorphouscarbonandSiO2(Fig.18).

Figure18.ThepanelontheleftshowsCOloadedalongwithPdspongeandthepanelontherightshowsCOwithNadispersedinthesamplechamber.Boththesampleswerephotographedatlowpressurespriortopolymerization.ThegasketmaterialusedinbothcasesisBe‐Cu.

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InteractionbetweenCOandnon‐metalssuchasCandSiO2butwithlargesurfaceareaofcontactsuchasglassy,nano‐carbon,silicawool,carbonnanotubesWhilenoappreciabledecreaseinpolymerizationpressureorchangesinthespectroscopiccharacteristicsofthepolymericphasewereobservedinmostcases,amodestreductioninthepolymerizationpressurewasobservedforamorphouscarbonandSiO2.Thesestudieshaveconsistentlyshownreductionofpolymerizationpressurebyat~1GPaandtheresultantpolymericphaseshowsallthesignaturesreportedpreviously.Aappreciable drop in polymerization pressure was also observed in the case of Li whenmediatedbyslightincreaseintemperature.WhileinthecaseofLi,thepolymerizationpressuredroppedto1GPa,inthecaseofNa,itdroppedto2GPawhenmediatedwithphoto‐reactivityofincident532nmor488nmlaserradiation.Whentheexperimentwasrepeatedusing660nmradiation,nochangeinpolymerizationpressurewasobserved.X‐ray diffraction of the polymerized products revealed a majority polymeric phase admixedwith a small amountof crystallinephase thatwasnotpureNaor Li indicative of a chemicalreaction (Fig. 19). This could imply that the polymerization is indeed aided by interactionbetween the alkali metals and CO and further that the activation energy involved could beloweredeitherbyincreasingthesurfaceofcontactortemperature(photo‐induced).

Figure19.TheintegrateddiffractionpatternsofthepolymericphaseobtainedwithLiat1GPaandNaat2.8GPa.Thebroadpeakweaksignalisdetectedinbothfromtheamorphouspolymer.ForLi,acrystallinephasewasobserved,whichindicatesacomplicatedlargeunitcellstructurenotsimilartoLiatthispressure(1.2GPa).