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Abstract Single crystals of CsGa ₇ O ₁₁ , RbGa ₇ O ₁₁ , and RbGa ₄ In ₅ O ₁₄ were grown from alkali halide melts and their structures were characterized by single crystal and powder X‐ray diffraction. CsGa ₇ O ₁₁ and RbGa ₇ O ₁₁ adopt the same structure type, reminiscent of the hollandite structure type, as it contains nearly rectangular channels made up of two dimers of edge‐sharing GaO ₆ octahedra, and two corner‐sharing octahedron/tetrahedron pairs. The structure of RbGa ₄ In ₅ O ₁₄ is more complex and is comprised of indium octahedra, gallium trigonalmore » bipyramids, and gallium tetrahedra, and contains similar sized tunnels as CsGa ₇ O ₁₁ and RbGa ₇ O ₁₁ . CsGa ₇ O ₁₁ and RbGa ₄ In ₅ O ₁₄ were further characterized by TGA, ion exchange experiments, and DFT studies revealing that both structures are thermodynamically stable up to 850°C; however, CsGa ₇ O ₁₁ decomposes to GaO(OH) xH ₂ O when heated in warm aqueous solutions. CsGa ₇ O ₁₁ undergoes ion exchange in both an aqueous solution of RbCl and a RbNO ₃ melt, as predicted by DFT studies, where the ion exchange is more extensive in the RbNO ₃ melt.« less

The flux synthesis of a uranium gallophosphate and a uranium gallate, Cs ₄ [UO ₂ Ga ₂ (PO ₄ ) ₄ ] and Cs ₂ UO ₂ Ga ₂ O ₅ , and 4 uranium phosphates, [Rb ₂ Rb _3.93 Cl _0.93 ][(UO ₂ ) ₅ (PO ₄ ) ₅ ], Rb ₁₁ [(UO ₂ ) ₈ (PO ₄ ) ₉ ], Rb _7.6 [(UO ₂ ) ₈ O _8.6 F _0.4 (PO ₄ ) ₂ ], and Rb ₆ [(UO ₂ ) ₅ O ₂ (PO ₄ ) ₄ ], is reported.

Topochemical reactions between CaCrO₃ and polyvinylidene difluoride yield the new fluorinated phase CaCrO_2.5F_0.5, which was characterized by powder synchrotron X-ray diffraction, X-ray photoemission spectroscopy, and magnetic susceptibility measurements. The reaction proceeds via reduced oxide intermediates, CaCrO_2.67 and CaCrO_2.5, 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-C₃N_4. Both CaCrO₃ and CaCrO_2.5F_0.5 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 bothmore » the ab plane and along the c-direction, which indicates an increased orthorhombic distortion due to the fluorination. Overall, while the oxygen vacancies are ordered in both intermediate phases, CaCrO_2.67 and CaCrO_2.5, a site preference for fluorine in the oxyfluoride phase cannot be confirmed. CaCrO₃ and CaCrO_2.5F_0.5 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 CaCrO_2.5F_0.5 structure.« less

The novel phases Rb₄NaU₃O_12-xF_x (1), K₄NaU₃O_12-xF_x (2), and Rb_2.1K_1.9KU₃O_12-xF_x (3) were synthesized by molten flux methods using mixed alkali fluoride melts. The oxyfluorides crystallize in the cubic space group Im[3 with combining macron]m with a lattice parameters of 8.7472(2) Å, 8.6264(2) Å, and 8.8390(3) Å, 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 uranylmore » 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.« less

Abstract 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.

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 $$I_{4_1}$$/amd with lattice parameters $$a$$ = 7.3254(2) and c = 30.9849(7) and is constructed from edge-sharing chains of UO₇ pentagonal bipyramids that are connected to [Al₂O₅]^4– two-dimensional sheets. The cesium cations, which are heavily disordered, occupy small channels in the $$a$$ and $$b$$ directions in the framework structure. The optical properties and ion exchange behaviors are reported along with DFT calculations that support the observed results of the ion exchange experiments.

Molten flux crystal growth is a thriving field for the discovery of uranium oxides.

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 thatmore » 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 $$U_{eff}$$ value. Volume-based thermodynamics was also applied to calculate the total energies of the different polymorphs, showing consistently higher stability of the layered polymorphs compared to the 3D polymorphs. For each of the studied polymorphs, we calculated the electronic, optical, and bonding properties. We also show an anisotropy in the absorption indexes along the three crystallographic directions of the polymorphs, which is especially noticeable in the layered polymorphs. We attribute the difference in the density of states to the different coordination of the U atoms in the layered and 3D polymorphs. We attribute the preferred formation of the 3D Cs polymorph to the substantial increase in the U–A bond strength, which is more pronounced than the differences in the bond strength between structures for other atomic pairs.« less