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The molten flux synthesis of a uranium gallophosphate and a uranium gallate, Cs4[UO2Ga2(PO4)4] (1) and Cs2UO2Ga2O5 (2), and four uranium phosphates, [Rb2Rb3.93Cl0.93][(UO2)5(PO4)5] (3), Rb11[(UO2)8(PO4)9] (4), Rb7.6[(UO2)8O8.6F0.4(PO4)2] (5), and Rb6[(UO2)5O2(PO4)4] (6), is reported. A systematic exploration of the UF4–GaPO4–ACl (A = Cs, Rb) phase space resulted in the synthesis of targeted Cs4[UO2Ga2(PO4)4] (1) and Cs2UO2Ga2O5 (2), which are gallium analogs of the previously reported aluminates, Cs4[UO2Al2(PO4)4] and Cs2UO2Al2O5. The exploration of this phase space simultaneously led to the synthesis and characterization of four new uranium phosphate phases: [Rb2Rb3.93Cl0.93][(UO2)5(PO4)5] (3), a salt inclusion material, and Rb11[(UO2)8(PO4)9] (4), both of which have complex 3D, porous, framework structures, and Rb7.6[(UO2)8O8.6F0.4(PO4)2] (5) and Rb6[(UO2)5O2(PO4)4] (6), both of which are layered structures related to the U3O8 topology. Fluorescence spectroscopy data are reported for all the compositions and are found to be typical for uranyl compounds.

Topochemical reactions between CaCrO3 and polyvinylidene difluoride yield the new fluorinated phase CaCrO2.5F0.5, which was characterized by powder synchrotron X-ray diffraction, X-ray photoemission spectroscopy, and magnetic susceptibility measurements. The reaction proceeds via reduced oxide intermediates, CaCrO2.67 and CaCrO2.5, in which CrO6 octahedral and CrO4 tetrahedral layers are stacked in a different manner along the c axis of CaCrO3. These two intermediate phases can be selectively synthesized by the carbothermal reduction with g-C3N4. Both CaCrO3 and CaCrO2.5F0.5 adopt the same orthorhombic space group, Pbnm; however, the fluorinated phase has decreased Cr-O-Cr bond angles as compared to the parent compound in both the ab plane and along the c-direction, which indicates an increased orthorhombic distortion due to the fluorination. While the oxygen vacancies are ordered in both intermediate phases, CaCrO2.67 and CaCrO2.5, a site preference for fluorine in the oxyfluoride phase cannot be confirmed. CaCrO3 and CaCrO2.5F0.5 undergo antiferromagnetic phase transitions involving spin canting, where the fluorination causes the transition temperature to increase from 90 K to 110 K, as a result of the competition between the increased octahedral tilting and the enhancement of superexchange interactions involving Cr3+ ions in the CaCrO2.5F0.5 structure.

The flux synthesis, solid state synthesis, and characterization of a new aluminate, Cs2(UO2)Al2O5, are reported. Cs2(UO2)Al2O5 crystallizes in the tetragonal space group I41/amd with lattice parame...

This frontier article focuses on the use of flux crystal growth for the preparation of new actinide containing 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, niobates, 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.

The novel phases Rb4NaU3O12−xFx (1), K4NaU3O12−xFx (2), and Rb2.1K1.9KU3O12−xFx (3) were synthesized by molten flux methods using mixed alkali fluoride melts. The oxyfluorides crystallize in the cubic space group Imm with a lattice parameters of 8.7472(2) A, 8.6264(2) A, and 8.8390(3) A, respectively. All three structures crystallize in a cubic perovskite structure, ABO3 (A4BB′3O12), where the A site is fully occupied by an alkali cation, and the B site is shared by the remaining smaller alkali cation and uranium in an ordered fashion such that the alkali cation on the B site is surrounded by square uranyl bipyramids. The structures were characterized by single crystal X-ray diffraction, energy dispersive spectroscopy, X-ray absorption near edge structure spectroscopy, X-ray photoelectron spectroscopy, magnetic susceptibility measurements, DFT calculations, thermogravimetric analysis, and UV-vis spectroscopy, all of which support the presence of U(V) in the three new materials.

Three new materials related to the phosphuranylite family were synthesized by using alkali chloride fluxes at 875 °C: CsNa3[(UO2)3O2(PO4)2] (1), Cs2Na4[(UO2)5O5(PO4)2] (2), and Rb6[(UO2)5O5(PO4)2] (3). CsNa3[(UO2)3O2(PO4)2] (1) crystallizes in the triclinic space group P1 with the lattice parameters, a = 6.9809(3) A, b = 9.3326(4) A, c = 12.9626(5) A, α = 71.5620(10)°, β = 78.9430(10)°, and γ = 68.0840(1)°; Cs2Na4[(UO2)5O5(PO4)2] (2) crystallizes in the triclinic space group P1 with the lattice parameters, a = 6.9890(3) A, b = 12.9652(6) A, c = 13.2086(6) A, α = 96.224(2)°, β = 101.433(2)°, and γ = 105.459(2)°; and Rb6(PO4)2[(UO2)5O5] crystallizes in the P21/m space group with the lattice parameters, a = 6.9255(3) A, b = 24.773(1) A, c = 7.07647(3) A, and β = 90.741(1)°. The sheets of 1 are based on the phosphuranylite topology, while the sheets of 2 and 3 contain sheets based on the U3O8 and uranophane topologies but differ in the orientation of the phosphate tetrahedra. The regions between the sheets ...

In this study we combine experimental synthesis and density functional theory (DFT) calculations to gain insight into the polymorphism of A4[(UO2)3(PO4)2O2] (A = Na, K, Rb, Cs) uranyl phosphate structures. Single crystals of a new 3D uranyl phosphate, Cs4[(UO2)3(PO4)2O2], were grown by molten flux methods using a CsCl flux. DFT calculations, using the DFT+U method, were carried out to study the difference between this new 3D uranyl phosphate and a family of recently described layered uranyl phosphates. Variation of the computed properties with changes in Ueff values are also studied. The DFT results agree with the experimental observations, showing that the Cs-containing 3D polymorph and the K-containing layered polymorphs are more stable than their respective layered and 3D polymorph. We show an increase in the difference between the total energies of the layered and 3D polymorphs and an increase in the band gaps with increasing Ueff value. Volume-based thermodynamics was also applied to calculate the to...

In this review we discuss general trends in the use of alkali bromide and iodide (ABI) fluxes for exploratory crystal growth. The ABI fluxes are ionic solution fluxes at moderate to high temperatures, 207 to ~1,300°C, which offer a good degree of flexibility in the selection of the temperature profile and solubility. Although their main use is to dissolve and recrystallize "soft" species such as chalcogenides, many compositions with "hard" anions, including oxides and nitrides, have been obtained from the ABI fluxes, highlighting their unique versatility. ABI fluxes can serve to provide a reaction and crystallization medium for different types of starting materials, mostly the elemental and binary compounds. As the use of alkali halide fluxes creates an excess of the alkali cations, these fluxes are often reactive, incorporating one of its components to the final compositions, although some examples of non-reactive ABI fluxes are known.

The title salt, (PMIM)2[Ni(P2S8)2] (PMIM = 3-methyl-1-propyl-1H-imidazol-3-ium, C7H13N2 +), consists of a nickel–thiophosphate anion charge-balanced by a pair of crystallographically independent PMIM cations. It crystallizes in the monoclinic space group P21/n. The structure exhibits the known [Ni(P2S8)2]2− anion with two unique imidazolium cations in the asymmetric unit. Whereas one PMIM cation is well ordered, the other is disordered over two orientations with refined occupancies of 0.798 (2) and 0.202 (2). The salt was prepared directly from the elements in the ionic liquid [PMIM]CF3SO3. Whereas one of the PMIM cations is well behaved (it does not exhibit disorder even in the propyl side chain), the other is found in two overlapping positions. The refined occupancies for the two orientations are roughly 80:20. Here, too, there appears to be little disorder in the propyl arm.

Topochemical reactions between CaCrO3 and polyvinylidene difluoride yield the new fluorinated phase CaCrO2.5F0.5, which was characterized by powder synchrotron X-ray diffraction, X-ray photoemission spectroscopy, and magnetic susceptibility measurements. The reaction proceeds via reduced oxide intermediates, CaCrO2.67 and CaCrO2.5, in which CrO6 octahedral and CrO4 tetrahedral layers are stacked in a different manner along the c axis of CaCrO3. These two intermediate phases can be selectively synthesized by the carbothermal reduction with g-C3N4. Both CaCrO3 and CaCrO2.5F0.5 adopt the same orthorhombic space group, Pbnm; however, the fluorinated phase has decreased Cr-O-Cr bond angles as compared to the parent compound in both the ab plane and along the c-direction, which indicates an increased orthorhombic distortion due to the fluorination. While the oxygen vacancies are ordered in both intermediate phases, CaCrO2.67 and CaCrO2.5, a site preference for fluorine in the oxyfluoride phase cannot be confirmed. CaCrO3 and CaCrO2.5F0.5 undergo antiferromagnetic phase transitions involving spin canting, where the fluorination causes the transition temperature to increase from 90 K to 110 K, as a result of the competition between the increased octahedral tilting and the enhancement of superexchange interactions involving Cr3+ ions in the CaCrO2.5F0.5 structure.

The title salt, (PMIM)

Single crystals of a new zinc oxysulfide SrZn2S2O were grown in a eutectic KF–KCl flux, and the structure was determined by single-crystal X-ray diffraction. SrZn2S2O crystallizes in the noncentrosymmetric polar space group Pmn21 with lattice parameters of a = 3.87440(10) A, b = 9.9847(3) A, and c = 6.0916(2) A. In the crystal structure, close-packed corrugated double layers of ZnS3O tetrahedra, which are derived from the wurtzite structure, are vertically separated by Sr2+ ions. In addition, the O/S anion ordered arrangement in each close-packed layer yields two distinct orientations of the Zn-centered tetrahedra. The crystals of SrZn2S2O are colorless and transparent, and the oxysulfide has a band gap of 3.86 eV, based on UV–vis–NIR diffuse reflectance measurements. Thermogravimetric measurements showed that SrZn2S2O is stable up to 650 °C in O2 gas atmosphere. First-principle calculations indicate that the valence band maximum is mainly composed of O-2p and S-3p states, whereas the conduction band mini...

Single crystals of eight new layered uranyl phosphates were grown from alkali chloride fluxes: Cs1.4K2.6[(UO2)3O2(PO4)2], Cs0.7K3.3[(UO2)3O2(PO4)2], Rb1.4K2.6[(UO2)3O2(PO4)2], K4[(UO2)3O2(PO4)2], K2.9Na0.9Rb0.2[(UO2)3O2(PO4)2], K2.1Na0.7Rb1.2[(UO2)3O2(PO4)2], Cs1.7K4.3[(UO2)5O5(PO4)2], and Rb1.6K4.4[(UO2)5O5(PO4)2]. All structures crystallize in the monoclinic space group, P21/c and contain uranyl phosphate layers with alkali metals located between the layers for charge balance. Ion exchange experiments on Cs0.7K3.3[(UO2)3O2(PO4)2], Rb1.4K2.6[(UO2)3O2(PO4)2], and K4[(UO2)3O2(PO4)2] demonstrated that Cs and Rb cations cannot be exchanged for K cations; however, K cations can be readily exchanged for Na, Rb, and Cs. Enthalpies of formation were calculated from density functional theory (DFT) and volume-based thermodynamics (VBT) for all six structures. A value for the enthalpy of formation of the phosphuranylite sheet, [(UO2)3O2(PO4)2]4–, was derived using single-ion additive methods coupled with VBT. DFT a...

The flux synthesis, solid state synthesis, and characterization of a new aluminate, Cs

The synthesis of four non-Lowenstein uranyl aluminophosphates, [Cs13Cl5][(UO2)3Al2O(PO4)6], Rb7[Al2O(PO4)3][(UO2)6O4(PO4)2], Cs3[Al2O(PO4)3][(UO2)3O2], and Rb3[Al2O(PO4)3][(UO2)3O2], the first uranyl phosphate salt-inclusion material [Cs4Cs4Cl][(UO2)4(PO4)5], and a related structure Cs4[UO2Al2(PO4)4], all prepared by molten flux methods, is reported. All compounds are discussed from the point of view of their structural features favoring, in some cases, ion-exchange properties. Lowenstein’s rule, well known in the realm of zeolites, aluminosilicate, and aluminophosphate minerals, describes the tendency of tetrahedra (Al, P, Si, and Ge) linked by an oxygen bridge to be of two different elements resulting in the avoidance of Al–O–Al bonds. Zeolites and related aluminosilicate/aluminophosphate minerals are traditionally formed under relatively mild temperatures, where zeolites are synthesized using the hydrothermal synthetic technique. Few exceptions to Lowenstein’s rule are known among aluminophosphates, an...

Two new metal thiophosphate anions, [Co(Co(P3S8)2)2]4− and [Cr(P3S9)2]3−, have been isolated as single crystals of EMIM (1-ethyl-3-methylimidazolium) salts from ionothermal reaction of the elements in [EMIM][CF3SO3]. The former compound is structurally unique from known thiophosphate anions, contains three cobalt ions in two different coordination environments and oxidation states, and has bridging, polydentate P3S83− groups that coordinate to the cobalt ions through both S and P ions. The chromium thiophosphate anion contains octahedrally-coordinated Cr ions and is structurally similar to the known ion [Ni(P3S9)2]4−. The structures of both salts were determined by single-crystal X-ray diffraction. Both compounds were characterized by EDX and IR spectroscopies.

Single crystals of 13 new uranyl germanate salt-inclusion materials were grown from alkali halide fluxes: [Cs2Cs5F][(UO2)3(Ge2O7)2] (1), [Cs6Ag2Cl2][(UO2)3(Ge2O7)2] (2), [Cs6Ag0.3Na1.7Cl2][(UO2)3(Ge2O7)2] (3), [Cs6Ag0.4Na1.6Cl2][(UO2)3(Ge2O7)2] (4), [Cs6K2Cl2][(UO2)3(Ge2O7)2] (5), [Cs6K1.9Ag0.1Cl2][(UO2)3(Ge2O7)2] (6), [KK6Cl][(UO2)3(Ge2O7)2] (7), [KK6Br0.6F0.4][(UO2)3(Ge2O7)2] (8), [Na0.9Rb6.1F][(UO2)3(Ge2O7)2] (9), [K0.6Na0.4K5CsCl0.5F0.5][(UO2)3(Ge2O7)2] (10), [K0.8Na0.2K4.8Cs1.2Cl0.5F0.5][(UO2)3(Ge2O7)2] (11), [KK1.8Cs4.2F][(UO2)3(Ge2O7)2] (12), and [Cs6Cs0.71Cl0.71][(UO2)3O3(Ge2O7)] (13). Structures 1–12 contain the same [(UO2)3(Ge2O7)2]6– framework whose pores are filled with varied salt species selected by the choice of the specific alkali halide flux used for crystal growth. The size and identity of the salt species also influence whether the [(UO2)3(Ge2O7)2]6– framework adopts a monoclinic or orthorhombic symmetry. The 13th composition, [Cs6Cs0.71Cl0.71][(UO2)3O3(Ge2O7)] (13), crystallizes in a n...

Formation enthalpies and Gibbs energies of actinide and rare-earth containing SIMs with silicate and germanate frameworks are reported. Volume-based thermodynamics (VBT) techniques complemented by density functional theory (DFT) were adapted and applied to these complex structures. VBT and DFT results were in closest agreement for the smaller framework silicate structure, whereas DFT in general predicts less negative enthalpies across all SIMs, regardless of framework type. Both methods predict the rare-earth silicates to be the most stable of the comparable structures calculated, with VBT results being in good agreement with the limited experimental values available from drop solution calorimetry.

Single crystals of eight new layered uranyl phosphates were grown from alkali chloride fluxes: Cs