DaltonTransactions

FRONTIER

Cite this: Dalton Trans., 2019, 48,3162

Received 26th November 2018,Accepted 20th January 2019

DOI: 10.1039/c8dt04675a

rsc.li/dalton

Flux crystal growth: a versatile technique to revealthe crystal chemistry of complex uranium oxides†

Christian A. Juillerat, ‡ Vladislav V. Klepov, ‡ Gregory Morrison,Kristen A. Pace and Hans-Conrad zur Loye *

This frontier article focuses on the use of flux crystal growth for the preparation of new actinide contain-

ing materials, reviews the history of flux crystal growth of uranium containing phases, and highlights the

recent advances in the field. Specifically, we discuss how recent developments in f-element materials,

fueled by accelerated materials discovery via crystal growth, have led to the synthesis and characterization

of new families of complex uranium containing oxides, namely alkali/alkaline uranates, oxychlorides,

oxychalcogenides, tellurites, molybdates, tungstates, chromates, phosphates, arsenates, vanadates, nio-

bates, silicates, germanates, and borates. An overview of flux crystal growth is presented and specific

crystal growth approaches are described with an emphasis on how and why they – versus some other

method – are used and how they enable the preparation of specific classes of new materials.

Introduction

The numerous recent literature reports of novel uranium con-taining materials with extended structures are the result ofmultiple parallel efforts to explore the crystal chemistry of

uranium containing materials, as well as of efforts aimed atimproving the nuclear fuel cycle and environmental remedia-tion projects. In addition, a recent push to develop newmaterials as a foundational basis for the development of newwaste forms that can be used to more effectively immobilizenuclear waste has also contributed.1 These combined effortshave resulted in an expansion of the synthetic approachesused to create novel structure types and have greatly increasedthe number of materials and structures containing uranium.The ability to obtain these materials in the form of single crys-tals has been instrumental for their discovery and the determi-nation of their structures. In fact, one can safely state that the

Christian A. Juillerat

Christian A. Juillerat graduatedwith a B. A. in Chemistry andPhysics from Lake Forest Collegein 2016. She is currently anInterdisciplinary GraduateEducation and Research Trainee(IGERT) fellow and is a graduatestudent in the laboratory ofDr Hans-Conrad zur Loye at theUniversity of South Carolina.Her research interests are in thebroad area of inorganicmaterials with a focus on thecrystal growth, structure determi-nation, and characterization ofnovel uranium oxides.

Vladislav V. Klepov

Vladislav V. Klepov graduatedwith a B.S. in Chemistry fromSamara State University in 2010and a Ph.D. in InorganicChemistry from LobachevskyUniversity of Nizhny Novgorod in2015 under the supervision ofDr Larisa B. Serezhkina. He iscurrently a post-doctoral fellowin the research group ofDr Hans-Conrad zur Loye at theUniversity of South Carolina. Hisresearch interest covers the chem-istry and crystal chemistry of

actinides, crystallography, crystal growth of novel compositions,and property measurements.

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c8dt04675a‡These authors contributed equally to the publication.

Department of Chemistry and Biochemistry, Center for Hierarchical Wasteform

Materials, University of South Carolina, Columbia, SC, 29208, USA.

E-mail: zurloye@mailbox.sc.edu

3162 | Dalton Trans., 2019, 48, 3162–3181 This journal is © The Royal Society of Chemistry 2019

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crystal growth process has been instrumental in the discoveryof the majority of these new materials.

This frontier article focuses on the use of flux crystalgrowth for the preparation of new actinide containingmaterials, reviews the history of flux crystal growth of uraniumcontaining phases, and highlights the recent advances in thefield. The overall focus is on specific crystal growth approachesthat are described with an emphasis on how and why they –

versus some other method – are used and how they enable thepreparation of specific classes of new materials, such asuranium containing oxides, fluorides or chalcogenides.2 Anattempt is made to realistically portray the underlying strat-egies of flux crystal growth experiments to create new compo-sitions that hopefully exhibit the desired structures, or that, byserendipity, exhibit something different but perhaps evenmore exciting. In many sections of this article, we highlightone interesting structure or structural feature within the dis-cussed class of compounds. More extensive discussions of thecrystal chemistry of uranium are available elsewhere.3–5

Likewise, the physical properties of uranium containing com-pounds which make them of particular interest, includingtheir luminescence and magnetic properties, are not discussedin this work but are discussed elsewhere.6–8

The use of fluxes for crystal growth, of course, goes backmany decades although only in the past two or three decadeshas flux crystal growth been applied in earnest to the prepa-ration of new uranium containing phases. Uranium contain-ing chalcogenides have been prepared via flux growth pri-marily by the Ibers group for some time and recent reviewssummarize this body of work; for this reason they are notcovered herein.9,10 Furthermore, many new uranium andthorium containing structures have been prepared byAlbrecht-Schmitt, Burns, Cahill, zur Loye, Obbade, Abraham,Loiseau, Dacheux, and Lii groups, among others, via hydro-thermal routes, a synthetic method that, although extremelyeffective, is outside the scope of this frontier article whichfocuses on flux crystal growth of uranium containing oxides,

including mixed anion oxides such as oxychlorides andoxychalcogenides.11–32,150–152

The preparation of a new material is not trivial as the tar-geted synthesis of an unknown compound is inherently chal-lenging. In some relatively simple systems, there exists exten-sive literature that one can use to make targeted predictions ofnew compositions and the expected structural variants. Forexample, in the case of perovskite oxides, a simple set of rulesexists to predict new compounds that will crystallize in the per-ovskite or related structure types. Unfortunately, this predic-tion becomes prohibitively difficult for more complex compo-sitions and structures, especially when the crystal chemistry isnot well-understood or the structure type is unknown. This iswhere flux crystal growth is incredibly useful as it facilitatesthe preparation of new complex compositions in the absenceof a fully established crystal chemistry; and, since the newcompositions are isolated as single crystals, their structurescan readily be determined. In general, the flux growth of newmaterials involves, first, the determination of conditionswhich are suitable for crystal growth within a specific system,including reactant identities, dwell temperatures, reactant con-centrations, and, importantly, the flux used. Once these con-ditions are determined, reaction conditions, especially reac-tant concentrations and dwell temperature, can be varied withthe goal of avoiding known phases and growing new com-pounds. With this in mind, in this frontier article we will focuson the flux crystal growth of complex uranium containingoxides, including, silicates, germanates, and phosphates, withan emphasis on the reactions conditions that are conducive tocrystal growth in these particular systems.

Flux crystal growth

The flux, typically an inorganic solid at room temperature,functions as the solvent at the high temperatures at whichcrystals are obtained via a conceptually well understood

Gregory Morrison

Gregory Morrison graduatedwith a B.S. in Chemistry fromthe University at Buffalo in 2009and a Ph.D. in Chemistry fromLouisiana State University in2013 under the supervision ofJulia Y Chan, where his researchcentered on rare earth contain-ing intermetallics. He is cur-rently a post-doctoral fellow inDr Hans-Conrad zur Loye’sresearch group at the Universityof South Carolina where hefocuses on the flux crystal

growth and chemistry of uranium and rare earth containingoxides.

Kristen A. Pace

Kristen Pace graduated with aB.S. in Chemistry from FloridaState University in 2015. She is aDOE Office of Science GraduateStudent Research (SCGSR) fellowand is currently a graduatestudent in Dr Hans-Conrad zurLoye’s research group at theUniversity of South Carolina.Her research is focused on thecrystal growth and characteriz-ation of complex uraniumoxides.

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sequence of events, beginning with nucleation and finishingwith growth.33,34 Our understanding of the step leading to thenucleation of specific phases, however, is still in its infancy,although we do understand, in general, the underlying reasonsfor nucleation to take place. In order for the nucleus, onceformed, to grow and not re-dissolve, it must reach a criticalsize that is a function of the degree of supersaturation. Ingeneral, the higher the degree of supersaturation, the smallerthe critical nucleus size can be. For this reason, it is preferredto use a flux that is able to dissolve a large quantity of thereagents and that exhibits a significant change in solubilitywith temperature to achieve this supersaturation. The nuclea-tion process has a temperature dependence and requires aminimum temperature for nucleation to occur and, further-more, has an optimum temperature range in which the nuclea-tion is at its maximum. Finding these optimum conditions,which tend to be unique for each system and change for evenminor compositional adjustments, is largely a matter of trialand error. Starting with a set of conditions that are known toyield crystals of a related composition or structure is a goodfirst approach and adjustments in temperature, concentration,time, reagent identity, etc., are made as needed.

Fluxes

The flux is the high temperature solution that functions as thesolvent for crystallization and, more often than not, consists ofa single, simple inorganic compound, such as B2O3, KCl, KOH,PbO, Bi2O3, or Na2CO3, which melt at conveniently low temp-eratures (Fig. 1). The combination of different solids to formeutectic compositions is one effective way to obtain an evenlower melting flux. A “good” flux has certain attributes includ-ing the ability to dissolve a significant quantity of the reagents,a large change of the solubility with temperature, a lowmelting point, low volatility, low cost and finally, being easilyremovable after crystal growth via dissolution in common sol-

vents, ideally water. There is no “universal flux” althoughmany fluxes can be used interchangeably and most crystalscan be grown out of more than one solvent system; there aresome advantages and disadvantages to all of them, however.

There are several chemical factors that influence the abilityof a flux to dissolve the reagents and to promote crystalgrowth. For example, materials that are good fluxes includethose that form a compound with the solute at lower tempera-tures or in different concentration ranges. The optimal concen-tration for flux crystal growth is individual to each system,although 1 : 10 molar ratio of reagents to flux is typically agood starting point. In addition, the presence of a commonanion or cation in flux and the reagent can have a positiveimpact on the solution chemistry and solubility as will match-ing the polarizability of the solvent and the solute. One impor-tant advantage of choosing the flux by matching the physicaland chemical properties is the formation of high quality crys-tals. A detailed introduction to fluxes is given in the review byBugaris and zur Loye.33

The reaction vessel used for crystal growth also is an impor-tant consideration, as many fluxes are highly reactive and willdissolve and/or chemically react with various containers.Therefore, one has to take into consideration the compatibilityof the reaction vessel and the flux. “Inert” containers includeplatinum and gold, which, however, are quite expensive and,in the case of gold, limit the crystal growth temperature to liebelow 1064 °C. Alumina crucibles are often used as a lessexpensive alternative and are considered chemically resistanttowards halide but not fluoride melts; however they can be dis-solved using hydroxide fluxes, leading to the incorporation ofaluminum into the product. Fluoride based fluxes cannot beused with alumina, as they readily react/dissolve alumina

Hans-Conrad zur Loye

Hans-Conrad zur Loye is theDavid W. Robinson PalmettoProfessor in the Department ofChemistry and Biochemistry atthe University of South Carolina.He received his B.S. from BrownUniversity in 1983 and his Ph.D.in Chemistry from UC Berkeleyin 1988. His research interestsfocus on the crystal growth ofnew materials, including newscintillating and luminescingoxides and fluorides, and newuranium and thorium containing

structures. In the latter case, new hierarchical wasteform materialsare synthesized for the effective immobilization of nuclear waste inpersistent architectures.

Fig. 1 The variety of fluxes used for obtaining complex uranium oxidesand their approximate operating temperature ranges.

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crucibles. Silver is a better alternative for both hydroxide andfluoride fluxes as silver is substantially inert towards them.Fused silica is a good choice for crystal growth experimentsnecessitating a sealed reaction environment, for example forgrowing crystals containing elements in reduced oxidationstates, as long as a silica compatible flux is chosen. Numerousother refractories and metal containers have been used appro-priately for the flux used and there is no universal containerthat is ideal for all flux growth experiments.

Uranium in extended structures exists predominantly in the+6 oxidation state, uranium’s most stable oxidation state inair, and the easiest to prepare. It is possible to prepare struc-tures containing uranium in the +4 and the +5 (rare) oxidationstates, but only if the syntheses are conducted under a con-trolled atmosphere and start with the appropriate, often pre-reduced, reagents. Depending on the target oxidation state,different fluxes should be used, for example carbonates andhydroxides for U(VI) containing materials and halides open toair for U(VI) and in sealed tubes for U(V) and U(IV) containingmaterials (Table 1). A more extensive version of Table 1 includ-ing the oxidation state of U, reagents, dwell time and tempera-ture, cooling rate and temperature, crystal color, reactionvessel and washing method is available in the ESI.† This isbest achieved by closely matching the bonding of the moltencompound (flux) to that of the final product i.e. ionic to ionic,covalent to covalent. Since most materials are not strictly oneor the other, compromises must be made. While it is possibleto use a flux in which the bonding is very different from thedesired product, the solubility of the reagents in such systemsis often quite limited.

Alkali, alkaline earth, and select mixedmetal uranates

The first synthesis and investigation of alkali metal uranatesdate back to the mid 20th century where the early sampleswere compositionally, but rarely structurally, characterized. Avariety of synthetic approaches were used to prepare thesephases, most of which resulted in polycrystalline samples thatwere structurally investigated by powder X-ray diffraction. Thedevelopment of single crystal X-ray diffraction promptednumerous research groups to pursue the growth of single crys-tals in order to precisely elucidate the structures of this classof materials and, beginning in the 1980s, numerous reportson the crystal growth and structural determination began toappear in the literature. These researchers typically relied onthe use of flux crystal growth to obtain the crystals needed forstructural determination and soon a significant number oflithium, sodium, potassium and especially cesium uranateswere grown as single crystals and structurally characterized.Thus, Wolf et al. published the flux crystal growth of alphaand beta Li6UO6, which were obtained by dissolving U3O8 orUO3, contained in a sealed gold capsule, in a Li2O2 flux formany days followed by slow cooling.81 The lemon-yellow crys-tals were obtained in good yield and were observed to change

to an orange color after a few hours of exposure to air.Subsequent flux crystal growth experiments on the otheralkali uranates suggest that these times are likely longer thanneeded as many flux growth experiments are complete after12–24 hours.

The sodium uranates exhibit more structural varieties whencompared to lithium, and have been obtained from a variety offluxes. Beta-Na2UO4 was crystallized by Gasperin using acomplex Na2CO3-Nb2O5 flux by heating U3O8 in this mixture at1190 °C for 15 hours.83 This composition crystallizes in aK2NiF4 related structure. Orange-yellow plate crystals ofNa4UO5 were grown by Roof et al. at much lower temperaturesout of a molten hydroxide flux at only 750 °C for 1 d followedby slow cooling. Using the same conditions, Roof et al. alsoobtained orange yellow plate crystals of K2UO4 (Fig. 2), whichcrystallize in the K2NiF4 structure.36 The hydroxide fluxes,which are known to be highly effective for the crystal growth ofoxides containing transition metals in high oxidation states,work exceedingly well for the synthesis of K2UO4 and Na4UO5,which readily crystallize out of hydroxide fluxes. Unfortunately,these two compositions are strongly favored no matter whatother elements are present in the melt, limiting the use ofhydroxides for the exploration of more complex uranium con-taining structures. Jove et al. grew orange crystals of Na2U2O7

out of a Na2CO3–Nb2O5 flux charged with U3O8 and orangecrystals of K2U2O7 out of a K2CO3–Nb2O5 flux charged withU3O8; in both cases the fluxes were heated to 1150 °C inair.59,133 The two compositions are isostructural with K2Np2O7.Read et al. crystallized a novel cubic phase, K8U7O24, in auranium deficient KUO3 structure, out of a KF flux containedin a copper tube at 950 °C followed by slow cooling. Theuranium source was UO2 and care was taken to eliminate waterand oxygen from the reaction. The reaction yielded a mixtureof yellow K2U2O7 crystals and black cubes of K8U7O24.

48 This isa rare example of an alkali uranate containing uranium in anoxidation state less than +6. A more complex potassiumuranate, K9U6O22.5 was crystallized by Saine et al., out of aK2CO3–Nb2O5 flux charged with U3O8.

57 The cubic structurecontaining all U(VI) differs from the all U(V) containing cubicKUO3 structure by virtue of the uranyl (UO2

2+) species thattetragonally distort the UO6 octahedra.

Rb4U5O17, was reported by Saad et al., who grew yellow crys-tals out of a U3O8 containing Rb2O–Sb2O3 flux by heating themixture at 1300 °C for 2 d followed by slow cooling to RT.55

This composition is isostructural to the cesium analog,Cs4U5O17. The cesium uranates are the most numerous onesamong the alkali uranates and were extensively investigated byEgmond, who grew numerous cesium uranate crystals forstructure determination, including yellow crystals of the afore-mentioned Cs4U5O17 as well as Cs2UO4, Cs2U4O12, Cs2U7O22,and Cs2U15O46 out of a CsCl flux.64,68 These structures tend tobe related to the UO2 structure where cesium atoms take theplace of the uranium. This work was extended by Morrisonet al., who reported the crystal growth of Cs2.2U5O16 andCs2U4O13, also from a CsCl flux.51 The structure of Cs2.2U5O16

is closely related to Cs2U5O16 via the addition of extra Cs to the

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Table 1 List of flux grown uranium oxides and the flux used organized by category

Compound Flux Ref. Compound Flux Ref. Compound Flux Ref.

Uranates Mixed Oxides OxychalcogenidesK2UO4 KOH 35 Ba2CuUO6 K2CO3 36 UYb2O2S3 Sb2S3 37Na4UO5 NaOH 35 Ba2NiUO6 K2CO3 36 UY2O2S3 Sb2S3 37Sr3UO6 K2CO3 36 Ba2ZnUO6 K2CO3 36 Na2Ba2((UO2)S4) Na2O2 38Ba2Na0.83U1.17O6 Na2CO3 39 MnUO4 CsCl 40 UY4O3S5 Sb2S3 41BaK4U3O12 K2CO3 39 FeUO4 BaCl2 40 UTa2O(S2)3Cl6 TaS2 42Na3Ca1.5UO6 Na2CO3 39 NiU2O6 KCl 40 UYb2O2Se3 Sb2Se3 43CaUO4 K2CO3 44 Cs2Mn3U6O22 CsCl 45 U2Pr2O4Se3 Sb2Se3 43β-Ca3UO6 K2CO3 44 K2MnU3O11 KCl 46 U2Sm2O4Se3 Sb2Se3 43K4CaU3O12 K2CO3 44 Rb2MnU3O11 RbCl 46 U2Gd2O4Se3 Sb2Se3 43K4SrU3O12 K2CO3 44 Li3.2Mn1.8U6O22 LiCl 46 U7O2Se12 CsCl 47K8U7O24 KF 48 Na4.5Nd0.5UO6 NaOH/KOH 49 UOS Sb2S3 50Cs2.2U5O16 CsCl 51 Pb3UO6 PbO 52 UOSe Sb2Se3 50Cs2U4O13 CsCl 51 β-CdUO4 CdCl2 53 UEr2O2S3 Sb2S3 54Rb4U5O17 Sb2O5 55 PbUO4 PbClF 56Ca(K6Ba2)U6O24 K2CO3 57 K9BiU6O24 B2O3 58 MolybdatesKUO3.5 (K2U2O7) KCO3 59 Oxychlorides Cs2((UO2)O(MoO4)) Cs2CO3 60Na8Ca3.11U3.7O17.52 Na2CO3, CaCO3 61 K4U5O16Cl2 KCl 62 Li4((UO2)10O10(Mo2O8)) Li2CO3 63Cs2U4O12 Cs2SO4 64 Rb4U5O16Cl2 RbCl 62 Cs6[(UO2)2(MoO4)3(MoO5)] Cs2CO3 65BaUO4 BaCl2 66 Cs5U7O22Cl3 CsCl 62 β-Cs2[(UO2)2(MoO4)3] Cs(COOCH3)/MoO3 67Cs4U5O17 CsCl 68 RbUO3Cl RbCl 62 Ag6[(UO2)3O(MoO4)5] AgNO3/MoO3 69MgUO4 MgCl2 70 CsUO3Cl CsCl 62 K2(UO2)2(MoO4)O2 K2CO3 71CaUO4 CaCl2 72 KUO3Cl KCl 73 K8(UO2)8(MoO5)3O6 K2CO3 71SrUO4 SrCl2 72 Csx(UO2)OClx CsCl 74 K2(UO2)(MoO4)2 MoO3 75α-Li2UO4 LiCl 76 Cs2UNb6Cl15O3 CsCl 77 Rb2U2MoO10 MoO3 78MgUO4 MgCl2 79 KUO3Cl KCl 73 (Cu, Mn)UMo3O12 MoO3 80β-Li6UO6 Li2O2 81α-Li6UO6 Li2O2 81 Tungstates 82 NiobatesNa2(UO4) Na2CO3 83 Bi2(W0.7U0.3)O6 K2SO4/Na2SO4 82 Cs9((UO2)8O4(NbO5)(Nb2O8)2) Cs2CO3 84Cs2U4O12 Cs2SO4 64 Bi2(W0.4U0.6)O6 K2SO4/Na2SO4 85 TlNb2U2O11.5 Tl2CO3 86K9U6O22.5 K2CO3 57 Rb8((UO2)4(WO4)4(WO5)2) Rb2CO3 85 UTiNb2O10 H3BO3 87

Cs8((UO2)4(WO4)4(WO5)2) Cs2CO3 85 TlNb2U2O11.5 Tl2CO3 86Tellurites Rb6((UO2)2O(WO4)4) Rb2CO3 88 KNbUO6 K2CO3 86K2[(UO2)3(TeO3)2O2] KCl 89 Li4((UO2)2(W2O10)) Li2CO3 90 RbNbUO6 Rb2CO3 86Rb2[(UO2)3(TeO3)2O2] RbCl 89 Na2Li8[(UO2)11O12(WO5)2] Li2CO3/Na2CO3 CsNbUO6 Cs2CO3 91Cs2[(UO2)3(TeO3)2O2] CsCl 89 92 KNbUO6 K2CO3 86Ca2(UO3)(TeO3)2 H6TeO6 93 Chromates 92 Nb7.6U2.4Ba5.2K0.8O30 K2CO3 94K2(UO2)2O2(TeO3) H6TeO6 93 Cs2(UO2)(CrO4)2 CsNO3K4[(UO2)5(TeO3)2O5] KCl 95 Rb2(UO2)(CrO4)2 RbNO3Silicates Germanates PhosphatesRb2USiO6 RbF 96 K4((UO2)Eu2(Ge2O7)2) MoO3-KF 97 Rb6[(UO2)7O4(PO4)4] RbCl 98Cs2USiO6 CsF 96 [Cs2Cs5F][(UO2)3(Ge2O7)2] CsF 99 Cs6[(UO2)7O4(PO4)4] CsCl 98K4CaUSi4O14 KF/CaF2 eu 100 [Cs6Ag2Cl2][(UO2)3(Ge2O7)2] CsCl 99 A4(PO4)2[(UO2)3O2] ACl 101K2USi6O15 KF/KCl eu 102 [Cs6Na2Cl2][(UO2)3(Ge2O7)2] CsF/CsCl/NaCl 103 Li6((UO2)12(PO4)8(P4O13)) P2O5 104Rb2USi6O15 RbF/RbCl eu 102 [Cs6AgxNa2−xCl2][(UO2)3(Ge2O7)2] CsF/CsCl eu 99 Li((UO2)(PO4)) P2O5 105Cs2USi6O15 CsF/CsCl eu 102 [Cs6K2−xAgxCl2][(UO2)3(Ge2O7)2] CsF/CsCl eu 99 Li2((UO2)3(P2O7)2) P2O5 105[NaK6F][(UO2)3(Si2O7)2] KF/NaF eu 106 [KK6Cl][(UO2)3(Ge2O7)2] KF/KCl eu 99 Cs3(UO2)2(PO4)O2 CsI 107[KK6Cl][(UO2)3(Si2O7)2] KF/NaF eu 106 [NaK6Cl][(UO2)3(Ge2O7)2] Na2CO3/KF/KCl 103 α-Ba2[UO2(PO4)2] B2O3 108K8(K5F)U6Si8O40 KF/KBr 109 [KK6Br0.6F0.4][(UO2)3(Ge2O7)2] KF/KBr eu 99 β-Ba2[UO2(PO4)2] H3BO3 108Cs2USi2O8 CsCl 110 [Na0.9Rb6.1F][(UO2)3(Ge2O7)2] RbF/NaF eu 99 α-K[(UO2)(P3O9)] P2O5 111[Cs3F][(UO2)(Si4O10)] CsCl 110 [KK1.8Cs4.2F] [(UO2)3(Ge2O7)2] CsF/KF eu 99 β-K[(UO2)(P3O9)] P2O5 111

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channels, while Cs2U4O13, seemingly related to the reducedCs2U4O12, crystallizes in a quite different structure.

A natural extension of the alkali uranates is both the alka-line earth uranates and mixed alkali–alkaline earth uranates.The latter can be crystallized out of carbonate melts, forinstance Ba2Na0.83U1.17O6, Na3Ca1.5UO6, and K4BaU3O12

reported by Roof et al., and the isostructural K4CaU3O12 andK4SrU3O12 reported by Read et al.39,44 In both cases, Na2CO3

and K2CO3 fluxes were used at temperatures ranging from900–1050 °C followed by slow cooling. These structures crystal-lize in perovskite related structures with fairly complex cationordering. The exception is Na3Ca1.5UO6, structurally related toNa4.5Nd0.5UO6, which crystallizes in a unique ordered rock saltsuperstructure in which NaO6, CaO6 and UO6 octahedra areedge and corner shared to create the overall 3D structure. Anextremely complex composition, Na8U2U0.81(U0.89Ca3.11)O17.35,(nominally a perovskite of the type A8B8O24) was reported byGasperin et al. in which U(VI) and Ca(II) occupy the same sitesin the structure.61 It is a mixed valence structure and containsa mixture of U(V) and U(VI) with an overall average oxidationstate of +5.5.

By comparison, the all-alkaline earth uranates are fairlysimple. They primarily form with the compositions of AUO4

and A3UO6, and have been reported for Mg, Ca, Sr, and Ba.MgUO4, CaUO4, and SrUO4 were reported by Zachariasen in1948 and 1954, representing some of the first uranate singlecrystal structures reported, and the barium analog wasreported by Reis et al. in 1976.66,70,72 The yellow green A2UO4

crystals were grown out of an MgCl2 CaCl2, SrCl2 or BaCl2melt, respectively, using U3O8 as the uranium source. Theseries is not isostructural due to the size differences of thealkaline earths resulting in structural distortions and the com-positions crystallize in a number of different space groups.T

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P 2O5

111

[Cs 2Cs 5F][(UO2) 2(Si 6O17)]

CsC

l11

0K2(U

O2)G

eO4

K2CO3/W

O3

103

Cs 1

1Eu 4(U

O2) 2(P

2O7) 6(PO4)

CsPO3/Cs 4P 2O7

112

[Cs 9Cs 6Cl][(UO2) 7(Si 6O17) 2(Si 4O12)]

CsC

l11

0K6(U

O2) 3Ge 8O22

K2CO3/M

oO3/PbO

103

(Nd 0

.38U0.62)(PO

3) 4

conc.H

3PO

411

3Rb 2(U

O2)Si 2O6

RbF

/RbC

leu

114

α-Cs 2(U

O2)G

e 2O6

Cs 2CO3/V

2O5/CsF

103

Rb 2(U

O2)(P 2O7)

Rb 4P 2O7

115

Cs 2(U

O2)Si 2O6

CsF/CsC

leu

114

β-Cs 2(U

O2)G

e 2O6

CsC

l/CsF

103

Cs 2(U

O2)(P 2O7)

CsC

l11

5(K

3Cs 4F)((UO2) 3(Si 2O7) 2)

CsF-KF

116

Cs 2(U

O2)G

eO4

Cs 2CO3/V

2O5/CsF

103

(UO2) 2P 6O17

Polyph

osph

oric

acid

117

(NaR

b 6F)((UO2) 3(Si 2O7) 2)

RbF

-NaF

116

Rb 2((UO2) 3(P

2O7)(P 4O12))

P2O5

118

(Ca 0

.5Na 0

.5) 2NaU

Si8O20

Na 2WO4

119

Arsen

ates

CsU

2(PO4) 3

CsC

l12

0Ag 6[(UO2) 2(As 2O7)(As 4O13)]

As 2O5

121

U(PO3) 4

conc.H

3PO

411

3Borates

Ag 6[(UO2) 2(AsO

4) 2(As 2O7)]

As 2O5

121

U(P

4O12)

conc.H

3PO

411

3LiBUO5

B2O3

122

Na 6[(UO2) 2(AsO

4) 2(As 2O7)]

As 2O5

121

CaB

2U2O10

B2O3

123

Li5((UO2) 13(AsO

4) 9(As 2O7))

As 2O5

104

Ni 7B4UO16

B2O3

124

Li((UO2) 4(AsO

4) 3)

As 2O5

104

UB2O6

B2O3

125

Li3((UO2) 7(AsO

4) 5O)

As 2O5

104

NaB

UO5

B2O3

126

Ba 4((UO2) 2(As 2O7) 3)

As 2O5

127

MgB

2UO7

B2O3

128

Ba 3((UO2) 2(AsO

4) 2(As 2O7))

As 2O5

127

Sr[(UO2) 2(B

2O5)O

]H

3BO3/Li 2B4O7

12Ba 5Ca((U

O2) 8(AsO

4) 4O8)

As 2O5

127

Cs 6[(UO2) 12(BO3) 8O3](H

2O) 6

B2O3

129

K2((UO2)As 2O7)

As 2O5

130

Rb 6[(UO2) 12(BO3) 8O3](H

2O) 6

B2O3

129

Li((UO2)(AsO

4))

As 2O5

105

K4Sr

4[(UO2) 13(B

2O5) 2(BO3) 2O12]

H3BO3/K

2B4O7

129

Li((UO2)(AsO

4))

As 2O5

105

(UO2)(B2O4)

Boric

Acid

131

Rb((U

O2) 2(As 3O10))

As 2O5

118

Ba 2[UO2(AsO

4) 2]

B2O3

108

Ba 4[(UO2) 7(U

O4)(AsO

4) 2O7]

As 2O5

132 Fig. 2 Structure of K2UO4 with uranium polyhedra in yellow, oxygen

atoms in red, and potassium atoms in purple.

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The double perovskites Ca3UO6 and Sr3UO6 (A2ABO6) wereobtained as single crystals from a Na2CO3 and K2CO3 meltwith CaCO3 and SrCO3 providing the Ca and Sr, respectively;UO2 or U3O8 was used to supply the uranium.36

The specific identity of the uranium source, other than forthe crystallization of phases containing reduced uranium,does not seem exceedingly critical for product formation. Thevariety of fluxes used, alkali and alkaline earth halides, alkalicarbonates, and alkali oxides, have all demonstrated theirability to dissolve a variety of uranium sources, such as UO2,U3O8 and UO3. From the compositions listed, it is clear that“in-fill” compositions to complete a number of the abovemen-tioned structural and compositional series should in all likeli-hood be readily obtained using one of the methods describedabove.

Related phases containing transition and main groupelements in place of the alkaline earth cations have been pre-pared in recent years. The series Ba2MUO6 with M = Cu, Ni, Znis isostructural to Sr3UO6 discussed above.36 Crystals of theseoxides were grown out of carbonate melts, as already used forSr3UO6. In all cases, the uranium is in the +6 oxidation stateand Cu, Ni and Zn are divalent. The compositionally, but notstructurally related Pb3UO6 was crystallized out of molten PbOand the crystals were mechanically extracted. The expectationwas that Pb3UO6 would be structurally related to Ba3UO6;however, the irregular coordination environment of the leadcation results in the formation of unique (UO5)∞ chains, yield-ing a novel structure type.52 Analogs to the AUO4 structure alsoexist with, however, in some cases exhibiting unique differ-ences in oxidation states.56 CdUO4, which could be obtainedout of a CdCl2 melt, is structurally related to the AUO4 series.

53

MnUO4, FeUO4 and NiU2O6 are an interesting series of tran-sition metal uranates that can be grown out of chloridefluxes.40 MnUO4 is isostructural to MgUO4; however, FeUO4 isnot. While MnUO4 contains Mn(II) and U(VI), FeUO4 containsFe(III) and U(V), making the latter a rare example of a U(V) con-taining material. Similarly, NiU2O6 also contains U(V), thistime alongside Ni(II).

More complex transition metal uranates can be crystallizedout of alkali chloride melts, specifically K2MnU3O11 (Fig. 3),Rb2MnU3O11, Cs2Mn3U6O22 and Li3.2Mn1.8U6O22, whose struc-tures are related to the mineral Natrotantite, Na2Ta4O11.

45,46

The potassium and rubidium containing quaternary uranatescontain U3O8 topological type layers connected via MnO6 octa-hedra and K+ cations, resulting in a layered intergrowth typestructure. The Cs containing material is more complex and isbased on U3O8 topological type layers connected via layers con-sisting exclusively of MnO6 octahedra and layers consistingentirely of Cs+ cations, resulting in a doubled unit cell withordered layers. This arrangement separates the MnO6 layers byover 13 Å, leading to two dimensional magnetic propertiesbest described as a frustrated spin lattice and ferromagneticlike ordering at 12 K.

Oxychlorides

Uranium oxychlorides were first studied in the 1960s and havecontinued to receive attention. Multiple synthesis techniqueshave been used to prepare these compounds including solidstate methods, flux growth, mild hydrothermal techniques,and most commonly, by crystallization from hydrochloric acidsolutions.

All of the flux grown uranium oxychlorides, of which thereare ten, have been synthesized using alkali chloride fluxes,typically in a sealed fused silica tube.74 For example, in thefirst reported flux growth by Allpress and Wadsley amorphousUO3 in excess CsCl was heated at 600 °C in a sealed glasscapsule for several weeks to form Csx(UO2)OClx (x ∼0.9).62,73,74,77,134 Read et al. reported that a sealed system andwell dried reactants were instrumental in the formation of oxy-chlorides over pure oxides.62 However, in rare instances, oxy-chlorides can form in open systems. KUO3Cl was synthesizedfrom a mixture of U3O8, CoCl2·6H2O, and KCl flux in a coveredalumina crucible heated to 900 °C, where the role ofCoCl2·6H2O was unclear but necessary.73 M7(UO2)8(VO4)2O8Cl(M = Rb, Cs) were obtained from the reaction of(UO2)3(VO4)2·5H2O with a large excess of MCl flux in a Pt cruci-ble at 750 °C.134

An interesting pair of uranium oxychlorides, A4U5O16Cl2(A = K, Rb) shown in Fig. 4, were synthesized from the reactionof UO3 and ACl flux in sealed fused silica tubes and contain2-D uranyl sheets composed of UO6, UO7, and UO4Cl2 polyhe-dra. In these compounds, the bulky Cl ligands are directed out

Fig. 3 Crystal structures of (a) U3O8, (b) K2MnU3O11, and (c) Cs2Mn3U6O22 where uranium polyhedra are in yellow, manganese in brown, and pot-assium and cesium ions in purple and pink, respectively.

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of the sheets forcing the uranyl oxygens to lie within the planeof the sheet thus forming sterically mediated cation–cationinteractions (CCI), i.e. coordination of uranium(VI) atoms bythe –yl oxygens of another uranyl groups.135,136 In contrast,RbUO3Cl contains 1-D uranyl chains of UO6, UO7, and UO4Cl2polyhedra. This 1-D topology allows both the bulky chlorideatoms and the uranyl oxygens to stick out of the chain and nocation–cation interactions exist in this structure.62

Oxychalcogenides

Uranium-containing oxychalcogenides obtained via fluxgrowth are not very numerous and almost exclusively havebeen reported by the Ibers group.37,43,47,50,54,137–139 Many moreoxychalcogenide compounds, however, were obtained via solidstate synthesis, a method which proved useful for the synthesisof this class of compounds. The solid state synthesis of oxy-chalcogenides requires very precise control over the oxygencontent in the system because of the high stability of bothUOS and UO2, which can compete with the targeted oxychalco-genide phase.47 Other oxychalcogenides were obtained seren-dipitously from reactions that were thought to be oxygen free;however, the resulting product contained oxygen that hadcome either from the reaction vessel (typically, evacuated fusedsilica tube) or from an impurity in the starting materials.139

One of the unique aspects of oxychalcogenide flux crystalgrowth is that fluxes were typically used for the recrystalliza-tion of a powder phase obtained via a solid state route, ratherthan from a mixture of dissolved simple starting materials,which is typically performed in the flux crystal growth of othersystems. The recrystallization approach limits the flux choicesas the flux must both be inert towards the target phase and yetdissolve a good amount of it at the same time. Anotherrequirement for the flux used in oxychalcogenide flux crystalgrowth is that it must be either easily separable from the targetphase (e.g. either the flux or the target phase should formlarge single crystals readily available for handpicking) or besoluble in the most common dry solvents, such as methanol,ethanol, DMF, acetonitrile, etc. A flux that meets both these

requirements is antimony chalcogenide Sb2Q3 (Q = S or Se),which has been employed successfully for the recrystallizationof oxysulfides and oxyselenides, respectively. Although thisflux is not readily soluble, it ends up in the form of large crys-tals that can be physically removed from the target product. Atypical quantity of this flux used for recrystallization is approxi-mately equal to the mass of the uranium-containing productphase.

For the purpose of discussing specific examples of oxychal-cogenide crystals grown by the flux method, it is worth startingwith the examples of UOS and UOSe. The structures of thesetwo compounds were deduced from PXRD patterns datingback to 1949, while the single crystal structures of these twocompounds were reported significantly later, when Bang Jinet al. recrystallized UOS and UOSe powders from Sb2S3 andSb2Se3 fluxes at temperatures of 1000 °C and 950 °C, respect-ively.50 Among the products, plate-like black crystals werefound to be the corresponding oxychalcogenides.

A number of other oxysulfide and oxyselenide phases havebeen obtained from the antimony chalcogenide flux, i.e. rare-earth element containing phases UYb2O2S3, UY2O2S3,UY4O3S5, UYb2O2Se3, U2Pr2O4Se3, U2Sm2O4Se3, U2Gd2O4Se3,and UEr2O2S3.

37,43,54 The synthesis of all of these, with theexception of UYb2O2S3, follows the abovementioned two-stepprocedure, i.e. all compounds were obtained via solid stateroutes and then recrystallized in a Sb2Q3 flux. An attempt atobtaining the UYb2O2S3 compound from KCl flux was made,but it was unsuccessful due to a reaction between the startingreagents and the flux, resulting in the formation of K3Yb7S12,which illustrates the importance of suitable flux selection.37

However, the use of 100 mg of CsCl flux in a reaction betweenuranium, SeO2, and Se resulted in crystals of U7O2Se12,showing no flux component incorporation into the finalproduct.47

Although antimony chalcogenides are typical fluxes foruranium oxychalcogenide synthesis, there is an interestingexample in which UCl4 plays the role of a flux. A reactionbetween TaS2 and an excess of UCl4 (1.6 : 1 ratio) in an evacu-ated fused silica tube at 610 °C, which is only 20 °C above themelting point of UCl4, afforded crystals of UTa2O(S2)Cl6 in an

Fig. 4 (a) The layered structure of A4U5O16Cl2 with the uranyl bonds of the CCIs shown in bold and (b) the 1-D structure of RbUO3Cl whereuranium polyhedra are shown in yellow, cesium atoms in pink, chlorine atoms in green, and oxygen in red.

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approximately 40% yield.139 The source of oxygen for theresulting product is not known for certain, but most likelyresulted from an oxide impurity in the TaS2 reagent.

Concluding this section, a unique example of a uranylsulfide compound, Na2Ba2(UO2)S4, highlights the importanceof using new, unconventional fluxes for obtaining crystals ofnew materials.138 A reaction between uranium, Na2O2, sulfur,and BaS, with an excess of the second component, Na2O2,results in single crystals of the phase Na2Ba2(UO2)S4. Thiscompound exhibits a unique [(UO2)S4]

6– unit (Fig. 5) andallows one to speculate that highly oxidative fluxes, such asNa2O2, do not necessarily oxidize sulfur and, at the same time,maintain uranium in the highest oxidation state.

Tellurites

Uranyl tellurites represent a unique class of materials thatexhibit an extensive array of structural topologies. While his-torically hydrothermal synthesis routes were the predominantmethods used to synthesize uranyl tellurites, flux crystalgrowth has more recently been shown to also be a viable syn-thetic route to uranyl tellurites. Moreover, it has enabledaccess to compositions that have revealed further structuraldiversity amongst the variety of phases in this class ofmaterials. Extended structures containing uranium(VI) areoften characterized by two-dimensional sheet structures as aresult of the presence of the “uranyl” UO2

2+ unit, which prefer-entially forms bonding interactions along the equatorial plane.Compounds containing both uranyl and a main groupelement with a stereochemically active lone-pair, however,tend to form one-dimensional structures. Despite this, thecompositions obtained via flux crystal growth deviate from thistrend, indicating that a more comprehensive understanding ofuranyl tellurite structural chemistry is desirable.

One such deviation from this structural trend is observed inA2[(UO2)3(TeO3)2O2] (A = K, Rb, Cs), a family of layered alkaliuranyl tellurites prepared by Woodward et al.89 Compositionsin the A2[(UO2)3(TeO3)2O2] family were synthesized by flux

crystal growth, where the combination of ACl (A = K, Rb, orCs), UO3, and TeO3 in a 4 : 4 : 2 molar ratio resulted in yellowneedle crystals when heated to temperatures between 650 °Cand 850 °C for anywhere from one to six days. The targetphases contain the A, U, and Te elements in a 2 : 3 : 2 molarratio, suggesting that there is an excess of both ACl salts andUO3. A wide temperature range was determined to be condu-cive to the formation of A2[(UO2)3(TeO3)2O2] crystals, althoughit was noted that reaction temperatures above 650 °C wereneeded to achieve the reduction of Te6+ to Te4+. The reactionwas found to be successful in either sealed silica ampoules oropen ceramic crucibles; however, sealed reactions resulted inproducts with improved crystal quality.

K4[(UO2)5(TeO3)2O5], a similar composition, was also syn-thesized by Woodward et al. using similar reaction con-ditions.95 Yellow prisms of K4[(UO2)5(TeO3)2O5] were grownfrom a mixture of KCl, UO3, and TeO3 in a molar ratio of16 : 3 : 2: (U : Te : K), which was placed into an alumina crucibleand heated to 800 °C for three days, followed by slow cooling.In contrast to the previous three reactions, a large excess ofKCl is used as a flux, leading to crystallization from a morediluted salt solution. As shown in Fig. 6, a staircase topologyanalogous to the aforementioned A2[(UO2)3(TeO3)2O2] series isobserved in the structure of K4[(UO2)5(TeO3)2O5]. The struc-tural similarities between the K4[(UO2)5(TeO3)2O5] and theA2[(UO2)3(TeO3)2O2] phases suggest that minor variations insynthetic parameters during the flux growth of uranyl telluritescould be an effective method to develop our understanding ofexisting trends in their structural chemistry.

As an alternative to the use of alkali metal salts as fluxes,Zadoya et al. investigated the use of a telluric acid flux to syn-thesize Ca2(UO3)(TeO3)2 and K2(UO2)2O2(TeO3), which exhibitan unusual tetraoxido core coordination and a new layer topo-logy, respectively.93 Mixtures of CaO or KCl, (UO2)(NO3)2·6H2O,and H6TeO6 were added to a platinum crucible in a molar ratioof 1 : 2 : 3 (Ca/K : U : Te) and heated to 700 °C and then slowcooled, resulting in yellow needles of the product phases. Inboth cases, telluric acid was present in excess, playing the roleof the flux.

To conclude, although the tellurite system offers a lot ofcompositional and structural flexibility, only a few fluxes havebeen employed to study this system so far. Further investi-gations on flux crystal growth in this system employing thealkali halide fluxes with tellurium oxide or telluric acid couldbe a promising direction for novel compositions. Also, it isimportant to note that almost no physical properties of thecompounds in this class were reported, representing anadditional impetus for the further development of this class ofmaterials.

Molybdates, tungstates, andchromates

To date, single crystals of uranium molybdates were obtainedeither from MoO3 or carbonate fluxes. The use of the MoO3

Fig. 5 Uranium(VI) coordination polyhedron in the structure ofNa2Ba2(UO2)S4.

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flux was found to be very convenient for obtaining single crys-tals of new molybdates as it functions as the molybdenumsource, has a relatively low melting point, 795 °C, and partiallyevaporates at high temperatures, thereby saturating the meltwith respect to uranium and prompting the growth of largesingle crystals. Alkali carbonates, such as K2CO3 and Cs2CO3,have also been used successfully for the growth of uraniummolybdates. In addition to dissolving the starting materials,the carbonate fluxes also serve as a source of the alkali cations,eliminating the necessity of adding another alkali source, suchas nitrates, to the reaction. A combination of the two fluxes, amixture of both alkali carbonate/nitrate and MoO3, has alsobeen shown to favor the growth of single crystals of uraniummolybdates. The latter is a more complicated process as itinvolves a reaction between the alkali salt and molybdenumoxide to form a low melting point molybdate which then func-tions as the flux. For example, the melting point of Na2MoO4 is687 °C, which is about 100 °C lower than the melting point ofMoO3, 795 °C. It is noteworthy that all uranyl molybdate com-pounds have been obtained in a temperature range850–1000 °C, above the melting point of molybdenum oxide.

Single crystals of Li4[(UO2)10O10(Mo2O8)], K2[UO2(MoO4)2],Rb2[(UO2)2MoO4O2], Cs2[(UO2)2(MoO4)3], and Ag6[(UO2)3O(MoO4)5], among others, have been obtained from MoO3 fluxwith an addition of a nitrate or carbonate alkali metalsource.63,67,69,78,140,141 It is noteworthy that several of thesereactions, although called solid state reactions by the authors,involve non-stoichiometric ratios of the starting reagents,enabling an excess component to function as a flux.140,141

Cs6[(UO2)2(MoO4)3(MoO5)], K2(UO2)2(MoO4)O2, andK8(UO2)8(MoO5)3O6, two of which possess MoO5 trigonal bipyr-amidal units (Fig. 7), are related examples where the use of alarge excess of alkali carbonates resulted in the crystal for-mation.65,71 Further investigation of related compositionswould be of interest and it is noteworthy that no uranium mol-ybdates have been reported grown from an alkali halide flux todate, suggesting that this might be an alternative approach forthe crystal growth of this class of materials.

Alekseev et al. reported the flux crystal growth of severaluranium tungstates, e.g. Na2Li8[(UO2)11O12(WO5)2], Li4[(UO2)2(W2O10)], A8[(UO2)4(WO4)4(WO5)2] (A = Rb, Cs) and Rb6[(UO2)O(WO4)4].

85,88,90 Crystals of these phases were obtained by heatinga mixture of uranyl acetate UO2(CH3CO2)2·2H2O, alkali carbonateA2CO3, and tungsten oxide WO3 in a platinum crucible atroughly 950 °C. There is no apparent excess of any of thereagents, suggesting that therefore there is no readily identifi-able flux in these systems; however, complex products, such asLi2(UO2)4(WO4)4, obtained along with the target phaseNa2Li8[(UO2)11O12(WO5)2] suggest that there are multiple com-peting processes occurring in the reaction mixture.90 It has beenshown by Charkin et al. that a eutectic K2SO4–Na2SO4 mixturecan be used to recrystallize uranium-containing tungstates.82 Noother fluxes have been used to probe the A2O–UO3–WO3 (A =alkali earth element) systems so far.

Given the tendency of the chromate(VI) groups to decom-pose at high temperatures, only fluxes with low melting points

Fig. 6 The staircase layers in the structures of K2[(UO2)3(TeO3)2O2] (left) and K4[(UO2)5(TeO3)2O5] (right).

Fig. 7 A layer consisting of UO2O5 and MoO5 polyhedra in the structureof K8(UO2)8(MoO5)3O6.

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can be employed to grow crystals of uranium chromates. Thereare two compounds reported to date, Rb2(UO2)(CrO4)2 andCs2(UO2)(CrO4)2, that have been grown at a temperature of270 °C from a CrO3 flux, which served as a source of the chro-mate anions for the target phases as well. However, even atthat low temperature CrO3 undergoes decomposition, contami-nating the sample with the highly inert Cr2O3. Recently, wehave demonstrated the successful application of eutecticnitrate fluxes with low melting points, to grow thorium fluor-ides,142 avoiding the formation of the inert ThO2. A similarapproach might be employed to grow crystals of additionalchromate phases.

Phosphates and arsenates

The flux synthesis of uranium phosphates and arsenates isa relatively new area of synthesis, considering that there areonly two publications prior to the turn of the century, ascompared to the alkali and alkaline uranates that garneredsignificant attention in the 1980s. The hydrothermal andsolution based syntheses of hydrated phosphates andarsenates received significant attention for their relation-ship to the uranium phosphate and arsenate mineral class,one of the largest for uranium containing minerals. By com-parison, the anhydrous phosphate and arsenate phases syn-thesized at high temperatures by various synthetic routes,including flux synthesis, have received comparatively lessattention. To date, the Depmeier and Alekseev groupshave published all of the reported uranium arsenatesand half of the phosphates synthesized by flux methods,with additional contributions to the expansion of thisclass of materials by the zur Loye, Obbade, and Ibersgroups.98,101,104,105,107,108,111–113,117,118,120,121,127,130,143

The general synthesis route for crystalizing uraniumarsenates established by Depmeier and Alekseev consists ofloading uranyl nitrate, As2O5, and nitrates or carbonates ofdesired cations, such as Li, Na, K, Rb, Ca, Ba, and Ag, into aplatinum crucible and heating to 780–850 °C followed by slowcooling at 5–7 °C h−1 to 50–60 °C. As2O5 functions as a verylow melting point (315 °C) flux for these reactions that pro-duced crystals of layered structures Ag6[(UO2)2(As2O7)(As4O13)],Ag6[(UO2)2(AsO4)2(As2O7)], Na6[(UO2)2(AsO4)2(As2O7)], threedimensional structures Li5((UO2)13(AsO4)9(As2O7)), Li((UO2)4(AsO4)3),Li3((UO2)7(AsO4)5O), Li((UO2)(AsO4)), Ba3((UO2)2(AsO4)2(As2O7)),K2((UO2)As2O7), Rb((UO2)2(As3O10)), and the 1-D chainstructure Ba4((UO2)2(As2O7)3).

104,105,118,121,127,130 Additionally,the layered Ba5Ca((UO2)8(AsO4)4O8) and three dimensionalBa4[(UO2)7(UO4)(AsO4)2O7] were prepared by methods similarto those described above, with an increased heating tem-perature of 1150 °C.127 Ba2[UO2(AsO4)2], the only reportedarsenate synthesised to use a non-As2O5 flux, was preparedby loading uranyl nitrate, barium nitrate, ammonium dihydro-gen arsenate, and B2O3 flux into a Pt crucible that washeated to 1000 °C, held for 5 h, and slow cooled to 300 °C at5 °C h−1. This structure consists of layers of isolated uranyl

bipyramids connected through corner sharing of arsenatetetrahedra.108

Among the structures mentioned above, the lithium uranylarsenates, Li5((UO2)13(AsO4)9(As2O7)), Li((UO2)4(AsO4)3), andLi3((UO2)7(AsO4)5O) exhibit the most interesting structures asthey contain cation–cation interactions which occur in less than2% of all U6+ containing materials.104 Li5((UO2)13(AsO4)9(As2O7))is constructed by well-defined tubular units that when unfoldedonto a 2D plane exhibit the ubiquitous uranophane topology.Li((UO2)4(AsO4)3) is composed of uranophane type chains andperpendicular chains parallel to the a direction and the b direc-tion which are connected through uranyl square bipyramids viacation–cation interactions to form a complex 3D structure.Li3((UO2)7(AsO4)5O), shown in Fig. 8, is very similar toLi((UO2)4(AsO4)3) with additional cation–cation interactionsbetween two uranyl pentagonal bipyramids of perpendicularuranophane-type chains.

Alekseev and Depmeier also synthesized several phosphates,Li6((UO2)12(PO4)8(P4O13)), Li((UO2)(PO4)), Li2((UO2)3(P2O7)2), α-K[(UO2)(P3O9)], β-K[(UO2)(P3O9)], K[(UO2)2(P3O10)], andRb2((UO2)3(P2O7)(P4O12)), using a method similar to the oneused for arsenates by simply substituting As2O5 with P2O5 thatmelts at 580 °C.104,105,111,118,143 The structures of these phos-phates are three dimensional where Li6((UO2)12(PO4)8(P4O13))and Li((UO2)(PO4)) are structurally related to Li5((UO2)13(AsO4)9(As2O7)) and Li((UO2)4(AsO4)3), while α-K[(UO2)(P3O9)],β-K[(UO2)(P3O9)], K[(UO2)2(P3O10)], and Rb2((UO2)3(P2O7)(P4O12)) all contain polyphosphate units which are not foundin minerals, and are uncommon among uranyl phosphates.The use of a B2O3 flux was successful for the synthesis of abarium uranyl arsenate, Ba2[UO2(AsO4)2], and was also success-

Fig. 8 The structure of Li3((UO2)7(AsO4)5O) which contains perpendicu-lar chains of pentagonal bipyramids as observed in the uranophanetopology connected by cation–cation interactions. Uranium polyhedraare shown in yellow, arsenate tetrahedra in gray, oxygen atoms in red,and lithium atoms in light blue.

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ful for the synthesis of α- and β-Ba2[UO2(PO4)2] which are com-posed of 1D chains and synthesized using uranyl nitrate,barium carbonate, and boron phosphate.108 The reactants wereloaded into a platinum crucible along with the B2O3 flux andwere heated to 1000 and 1300 °C, respectively, held for 2 h andslow cooled to 300 °C at 7 °C h−1.

The reported phosphate flux syntheses utilized a morediverse collection of fluxes when compared to the arsenates,including the previously discussed P2O5, H3PO4 acid, polypho-sphoric acid, a mixed CsPO3/Cs4P2O7 flux, and alkali halidefluxes. P2O5 is difficult to weigh out in air considering the factthat it is extremely hygroscopic and reacts with water to formphosphoric acid. Diammonium phosphate and dihydrogenammonium phosphate are easier solids to weigh out, althoughit is important to note that both of these reagents ultimatelydecompose into metaphosphoric acid (as do phosphoric andpolyphosphoric acid) to form a glassy melt that acts as a flux;thus, fluxes of P2O5 vs. diammonium phosphate or dihydrogenammonium phosphate are significantly different. The use ofthe phosphoric and polyphosphoric acid fluxes allowed theuse of low reaction temperatures of 500 °C and 400–420 °C,respectively. (Nd0.38U0.62)(PO3)4, U(PO3)4, and U(P4O12) wereproduced from phosphoric acid fluxes where the former weremade from the reaction of uranyl nitrate, neodymium nitrate,and sodium carbonate, while the latter was synthesized fromU3O8.

113 It is interesting to note that while all of the otherphosphates and arsenates synthesized in air result in com-pounds containing U6+, those synthesized using the phospho-ric acid flux resulted in U(IV) containing products. The use ofthe polyphosphoric acid flux and the UO3 reagent produced aU(VI) containing compound, (UO2)2P6O17,

117 suggesting thatpolyphosphoric acid is not as reducing towards U(VI) as phos-phoric acid, which is interesting considering that both startingreagents effectively result in a metaphosphoric acid flux;however, it would be difficult to determine when during thedecomposition of the phosphorus containing reagent theproduct forms. U(IV) phosphates can also be synthesized bycombining UP2, Se, and CsCl flux in a sealed silica tube,heating to 950 °C and dwelling for 192 h before cooling to550 °C in 120 h. This synthesis reported by the Ibers groupproduced blue-purple dichroic crystals of CsU2(PO4)3.

120 Usingthe mixed CsPO3/Cs4P2O7 flux with UO3 and Eu2O3 startingmaterials in a platinum crucible, heated at 600 °C for 2 days,and 850 °C for 10 days, and slow cooling to 720 at 1 °C h−1

resulted in orange, luminescent crystals ofCs11Eu4(UO2)2(P2O7)6(PO4), reported by Pobedina et al.112

Alkali halide fluxes have also been used in reaction vesselsopen to air. The Obbade group reported the synthesis ofCs3(UO2)2(PO4)O2 obtained by using (UO2)3(PO4)2(H2O)4 as theuranium and phosphate source and CsI as the flux.107 Thesepowders were loaded into a platinum crucible in a 1 : 30 ratioand heated at 750 °C for 10 h before slow cooling at 7 °C h−1.The zur Loye group utilized UF4, AlPO4, and alkali chloridefluxes and heated mixtures with general molar ratios of3 : 2 : 120 in alumina crucibles at 875 °C for 12 h beforecooling to 400 °C at 6 °C h−1. The excess flux is washed away

with water to yield yellow or orange crystals ofA6[(UO2)7O4(PO4)4] (A = Rb, Cs), A4[(UO2)3O2(PO4)2] (A = K, Rb/K, Cs/K), and A6[(UO2)5O5(PO4)2] (A = Cs/K, Rb/K).98,101 Whilethe latter two families adopt layered structures of known topol-ogies, the A6[(UO2)7O4(PO4)4] structure type features chains ofphosphuranylite type units connected into 2D sheets throughuncommon uranyl cation–cation interactions.

Vanadates

Numerous vanadates have been obtained via hydrothermal syn-thesis. The higher temperature flux crystal growth, however,can make compositions accessible that cannot readily be pre-pared via hydrothermal methods; specifically, anhydrous com-positions are more conveniently obtained by flux crystalgrowth. The work by Abraham et al.27,144–146 which resulted ina large number of anhydrous vanadates via halide flux crystalgrowth is one example. They used a KCl flux at temperaturesjust above its melting point (reaction temperature 775 vs.770 °C melting point) to dissolve (UO2)3(VO4)2·5H2O and toobtain single crystals of K6(UO2)5(VO4)2O5.

144 A related compo-sition, Na6(UO2)5(VO4)2O5, was obtained by melting Na2UO4

and V2O5 in a 3 : 2 molar ratio at a temperature of 800 °C. Inthis crystal growth reaction, one can consider the excess V2O5

with a melting point of 690 °C to function as the flux. Singlecrystals of two polymorphs of the rubidium analog, α- andβ-Rb6(UO2)5(VO4)2O5,

145 were also obtained; the former wascrystallized from a RbI flux at a temperature 8 °C above the fluxmelting point of 642 °C, and the other one from a mixture ofU3O8 and a V2O5/Rb2CO3 flux in a 0.67 : 1 : 2 molar ratio at atemperature of 1200 °C. In the latter case, the excess of Rb2CO3

and V2O5 can be considered as a flux. Similarly, single crystalsof Na(UO2)4(VO4)3 were obtained by melting a mixture of V2O5,U3O8, and Na2CO3 in a 12 : 11 : 3 molar ratio.146

Alkali halides or vanadium(V) oxide, as in the previousexamples, are rather intuitive choices for a flux employed tocrystallize uranyl vanadates. A less conventional approach isthe synthesis resulting in [La(UO2)V2O7][(UO2)(VO4)], whichwas obtained by melting LaCl3·7H2O and U2V2O11 in a2 : 3 molar ratio at 870 °C.27 The structure of this compoundrepresents a 3D structure resulting from the stacking of[La(UO2)V2O7]

+ double layers and sheets of [(UO2)(VO4)]−.

Niobates

In addition to high-temperature solid-state methods, fluxgrowth has been used to synthesize several uranium niobates.Chevalier and Gasperin first reported on UTiNb2O10, whichthey obtained in single crystal form from a mixture of U3O8,Nb2O5, and TiO2 heated in a H3BO3 flux at 1200 °C.87 Gasperinlater reported on the use of carbonate fluxes to prepareTlNb2U2O11.5, KNbUO6, and RbNbUO6, which were grown inplatinum crucibles heated to temperatures of 1150, 1300, and1200 °C from mixtures of U3O8, Nb2O5 and Tl2CO3 or K2CO3/

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Rb2CO3 in a 1 : 1 : 2 and 1 : 1 : 6 molar ratio (U : Nb : Tl, K/Rb),respectively.86 CsNbUO6 was later reported by Gasperin whosynthesized it using methods identical to those used toprepare KNbUO6 and RbNbUO6, only changing the molar ratioof U3O8, Nb2O5 and Cs2CO3 to 1 : 1 : 2.5.86

Similarly, a mixed alkali/alkaline earth metal uraniumniobate (Nb7.6U2.4)(Ba5.2K0.8)O30 was prepared by Saine using aK2CO3 flux, representing an unusual composition in which theU(VI) site is surprisingly shared by Nb(V) and Nb(IV).94 Crystalsof this composition were grown from a 1 : 1 : 1 mixture ofU3O8, Nb2O5 and BaCO3 heated to 1250 °C for fifteen hours.

Silicates and germanates

Uranium silicates constitute an abundant class of materialswith over 60 compounds known, including nearly 20 minerals.Many of the synthetic uranium silicates have been grownunder hydrothermal conditions which mimic the conditionsunder which natural minerals often form. However, in the pastdecade, the flux growth of uranium silicates has received con-siderable attention and resulted in the discovery of 19 newcompounds.96,100,102,106,109,110,114,116,119

Alkali fluoride containing fluxes have been found to be veryeffective fluxes for the growth of complex uranium silicatesand all but one of the syntheses reported in the literature usesuch a flux. However, single species fluxes have had limitedsuccess, with KF typically producing a layered potassiumuranium oxide that decomposes when exposed to water duringflux removal and RbF and CsF preferentially crystallizingRb2USiO6 and Cs2USiO6, respectively.

114 For this reason, theflux crystal growth of uranium silicates has almost exclusivelybeen conducted in mixed alkali fluoride containing fluxes,most commonly AF–BF and AF–ACl fluxes (A, B = alkalimetals), where the fluoride is important to facilitate SiO2 dis-solution in the melt. For example, Rb2(UO2)Si2O6 was grown byheating a mixture of UF4 and SiO2 in a RbF/RbCl flux at800 °C. In a couple of instances, more complex alkali fluoridefluxes were used. Namely, K6(UO2)3Si8O22 was synthesizedfrom UO3 and SiO2 in a KF/KVO3 flux at 700 °C and

K4CaUSi4O4 was synthesized from the reaction of U3O8 andSiO2 in a KF/CaF2 flux at 900 °C.100,147

The one example of a non-alkali fluoride containing flux inthe growth of uranium silicates is the synthesis of(Ca0.5Na0.5)2NaUSi8O20, which was obtained from the reactionbetween UO2 and CaSiO3 in a Na2WO4 flux at 730 °C.119 Thissynthesis is also notable in that it produces a U(IV) silicate. Toprevent the oxidation of the uranium during flux growth, thereaction mixture was loaded into a flame-sealed, evacuated,carbon coated, fused silica ampoule. Three other U(IV) silicates,A2USi6O15 (A = K, Rb, Cs), have been obtained by flux growth.102

These were grown via the reaction of UO2, SiO2 and an AF/AClflux in sealed Cu tubes. The copper tubes had to be heated in aN2 flow to prevent the copper from oxidizing; the copper tubesare, however, relatively inert to fluoride fluxes which is not thecase with the more prevalent fused silica tubes.

One noteworthy class of materials amongst the uraniumsilicates are the salt-inclusion materials, SIMs. These materialsconsist of a covalent metal oxide framework that surrounds anionic salt lattice, often an alkali halide. While many varietiesof SIMs are known, the uranyl silicate SIMs are particularlyabundant. The first reported uranyl silicate SIMs,[NaRb6F][(UO2)3(Si2O7)2] and [K3Cs4F][(UO2)3(Si2O7)2], werecrystallized from a mixture of UO3 and SiO2 in a NaF/RbF orKF/CsF flux heated to 750 °C in a Pt crucible.116 Later researchfound that limiting the availability of oxygen to the melt byreducing the reaction surface area and by using halide asopposed to oxide precursors promoted the formations of SIMs.Using these enhancements, 7 more uranyl silicate SIMs werediscovered. For example, [Cs3F][(UO2)(Si4O10)] was grown at800 °C from a mixture of UF4, SiO2, and a CsF/CsCl flux in ahigh aspect ratio cylindrical Ag crucible (Fig. 9).106,110

Uranium germanates are considerably less well studiedthan their silicate counterparts. However, three recent papershave reported their flux growth and thereby greatly increasedthe number of known uranium germanates. In general, theflux synthesis of germanates is similar to that of silicates. Onenotable difference is that GeO2 is able to be solubilized bychloride melts, so fluoride free alkali halide fluxes can beused. For example, [Cs6Ag2Cl2][(UO2)(Ge2O7)], a SIM that crys-

Fig. 9 Optical images of crystals and framework structure of [Cs3F][(UO2)(Si4O10)], where uranium polyhedra are in yellow, silicate tetrahedra inblue, cesium atoms in pink, chlorine atoms in green, and oxygen atoms in red.

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tallizes in the same structure type as some of the silicate SIMs,was synthesized from the reaction of UF4 and GeO2 in a CsClflux heated to 875 °C in a silver crucible. Eleven other com-pounds with the same uranyl germanate framework anddifferent salt-inclusions were synthesized by varying the com-position of the flux.99 The second reported flux synthesis of auranyl germanate was that of K4[(UO2)Eu2(Ge2O7)2], which wassynthesized from UO3, Eu2O3, and GeO2 in a KF/MoO3 flux at950 °C.97 Finally, the synthesis of seven open-frameworkuranyl germanates, including two SIMs, was reported from thereaction of uranyl nitrate and GeO2 in a variety of fluxes,namely K2CO3–WO3, K2CO3–MoO3–PbO, Cs2CO3–V2O5–CsF,CsF–CsCl, Na2CO3–KF–KCl, and CsF–CsCl–NaCl.103

Borates

The relevance of uranium borates has primarily resulted fromresearch efforts focused on developing an understanding ofthe behavior of actinides in nuclear waste disposal processesin which, under certain process conditions, crystalline regionstended to form within borosilicate glass matrices.154

Hydrothermal synthesis methods have proved to be highlyeffective for the preparation of uranium borates and manyexamples of these compounds have been synthesized from lowtemperature boric acid fluxes using mild hydrothermalmethods by the Albrecht-Schmitt and Alekseev groups;however, this extensive body of work has been reviewed else-where.148 To date, the application of high temperature,ambient pressure flux growth techniques to the preparation ofuranium borates has resulted in several compounds, themajority of which have been prepared from B2O3 melts.

The first of these compounds was reported by Hoekstra, inwhich single crystals of UO2(BO2)2 were prepared by reactingU3O8 with an excess of B2O3 at approximately 1100 °C.153 It waslater reported by Holcombe and Johnson that polycrystallinepowders of the same composition could be obtained usingammonium diuranate as the uranium source in an excess ofB2O3; single crystals were then obtained by heating the powderto 850–900 °C in a B2O3 flux.125 The first alkali and alkalineearth metal uranium borates, Li(UO2)(BO3), Na(UO2)(BO3), Mg(UO2)B2O5, and Ca(UO2)2(BO3)2, reported by Gasperin, wereobtained from mixtures of U3O8 and the respective alkali/alka-line earth carbonate salts heated in B2O3 melts at temperaturesbetween 1100 and 1200 °C.122,123,126,128

Nearly three decades after Gasperin’s successful prepa-ration of several uranium borates using B2O3 as a flux,Alekseev et al. reported on the use of a mixed Li2B4O7/H3BO3

flux to synthesize Sr[(UO2)2(B2O5)O] from (UO2)(NO3)2·6H2Oand Sr(NO3)2 precursors heated to 965 °C.12 It was noted thatattempts to prepare Sr[(UO2)2(B2O5)O] by substitution ofLi2B4O7 with increased stoichiometric ratios of H3BO3 wereunsuccessful, leading the authors to deduce that addition ofLi2B4O7 is necessary for the stabilization of this phase.Attempts were also made to prepare the barium analogueusing Ba(NO3)2 as a precursor; however, this phase could only

be synthesized from mild hydrothermal methods. Alekseevet al. later reported on A6[(UO2)12(BO3)8O3](H2O)6 (A = Rb, Cs)and K4Sr4[(UO2)13(B2O5)2(BO3)2O12], synthesized using B2O3

and mixed K2B4O7/H3BO3 fluxes, respectively.129 TheA6[(UO2)12(BO3)8O3](H2O)6 compounds were prepared frommixtures of (UO2)(NO3)2·6H2O and the respective alkalimetal nitrate salt heated to 1000 °C, whileK4Sr4[(UO2)13(B2O5)2(BO3)2O12] was prepared from a mixture of(UO2)(NO3)2·6H2O and SrCO3 heated to 980 °C. Since both theA6[(UO2)12(BO3)8O3](H2O)6 phases have been obtained at ahigh temperature, it is very likely that the hydration of thesecompounds took place during dissolving the flux with water,which also explains the low quality of the crystals. AlthoughB2O3 melts have resulted in many compounds, this recentwork by the Alekseev group demonstrates the viability of pre-paring novel uranium borate phases from variable boratefluxes and, thus, highlights a potential area for future work.

Summary and outlook

Understanding the crystal chemistry of uranium and otheractinides has become more pressing as efforts to enhance thenuclear fuel cycle, studies to create new materials in which toimmobilize nuclear waste, and investigations to improveenvironmental remediation projects are being expected toemerge with new materials that can help solve existing pro-blems. To address these and other issues, over the pastdecades, numerous research groups have used fluxes to growcrystals of diverse classes of uranium containing materials,and in the process have established flux growth as an impor-tant tool for materials chemistry and for improving our under-standing of the crystal chemistry of uranium in general.Clearly, the utility of fluxes for material discovery of uraniumcontaining materials via exploratory crystal growth has beenand continues to be a rewarding undertaking.

The challenge of material discovery is present in all areas ofmaterials chemistry where the desire to find new or createchemically modified versions of materials that exhibit desiredstructures and physical behavior has driven research for manydecades. The use of fluxes for material discovery has beenreviewed by a number of investigators, each one typically focus-ing on a discrete area of materials chemistry, ranging fromintermetallics to complex oxides.149 In this frontier article wefocused on providing an overview of the history of flux crystalgrowth of uranium containing phases, and highlighting therecent advances in the field, limiting ourselves to uraniumcontaining oxides, including mixed anion oxides.

In order to grow crystals of desired materials, such asuranium containing phases, including quaternary and higherphases, the flux, by necessity, must be able to solubilize simul-taneously a diverse mix of oxide reagents. We highlighted anumber of fluxes that work eminently well for uranium con-taining oxides, including phosphates, silicate, borates, andarsenates; nonetheless, it is important to keep in mind thatsimple fluxes may not be up to this task, and for this reason

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the use of more complex fluxes may be required to targetphases that are more compositionally demanding. It will be upto the researchers to adapt and modify existing fluxes for thispurpose and, ideally, this frontier article will provide a usefulstarting point. The crystal growth from high temperature solu-tions is an adaptive field and we expect that new flux combi-nations will come into use and lead to the growth of additionalnew uranium containing complex oxides.

Finally, there remains the ever present tension between thecrystal size and crystal growth throughput. It is thereforeimportant to keep in mind that adapting a growth process thathas yielded a desired phase to one that produces multi milli-meter crystals of the same phase is on the one hand very timeconsuming, yet on the other hand very rewarding for theresearch opportunities that are opened up.

The rapid progress enabled by flux crystal growth, especiallyin the last few years, is expected to motivate other groups toalso utilize fluxes for material discovery, not only in uraniumcontaining materials, but in a wide range of material classesand to further expand our knowledge of crystal chemistry inthese areas.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported as part of the Center for HierarchicalWaste Form Materials, an Energy Frontier Research Centerfunded by the U.S. Department of Energy, Office of Science,Basic Energy Sciences, under Award DE-SC0016574.

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97 S.-P. Liu, M.-L. Chen, B.-C. Chang and K.-H. Lii, FluxSynthesis, Crystal Structure, and Photoluminescence of aHeterometallic Uranyl-Europium Germanate withU=O−Eu Linkage: K4[(UO2)Eu2(Ge2O7)2], Inorg. Chem.,2013, 52, 3990–3994.

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106 G. Morrison and H.-C. zur Loye, Flux Growth of[NaK6F][(UO2)3(Si2O7)2] and [KK6Cl][(UO2)3(Si2O7)2]: TheEffect of Surface Area to Volume Ratios on ReactionProducts, Cryst. Growth Des., 2016, 16, 1294–1299.

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