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A systematic investigation is presented which examines the structural chemistry of anhydrous and hydrous ternary hexavalent uranium and neptunium dinitrato, trinitrato, and tetranitrato complexes. Using slow evaporation methods under acidic conditions the uranium and neptunium nitrate complexes γ-K[UO2(NO3)3], K2[UO2-cis-(NO3)4], [NpO2(NO3)2(H2O)2]·4H2O, and Cs[NpO2(NO3)3] have been synthesized and their structures refined using single-crystal X-ray diffraction data. γ-K[UO2(NO3)3] adopts an orthorhombic structure in space group Pbcaconsisting of antiparallel aligned [UO2(NO3)3]−moieties. K2[UO2-cis-(NO3)4] adopts a monoclinic structure in space group P21/cconsisting of [UO2(NO3)4]2–moieties with two monodentate and two bidentate nitrate ligands that are arranged in a cisconfiguration about the uranyl, UO22+, center. Previous investigations have only identified transvariants of this monoclinic structure, and this is the first report of the cisform and also the occurrence of geometric isomerism in uranyl nitrates. [NpO2(NO3)2(H2O)2]·4H2O adopts an orthorhombic structure in space group Cmc21consisting of parallel aligned [NpO2(NO3)2(H2O)2] moieties that are in a transconfiguration with respect to the bidentate nitrate ligands. Cs[NpO2(NO3)3] adopts a hexagonal structure in space group R3cconsisting of parallel aligned [NpO2(NO3)3]−moieties. It was found that despite using a Np(V) nitrate solution as the starting reagent, Np(VI) nitrate structures were consistently recovered under acidic conditions. These observations are discussed and rationalized with respect to standard reduction potentials, particularly how redox conditions and acidity affect the oxidation state of Np and subsequent structure formation. The structures uncovered in this investigation are discussed comparatively and systematically in detail with other reported anhydrous and hydrous ternary hexavalent uranium and neptunium dinitrato, trinitrato, and tetranitrato complexes, particularly with respect to how synthesis conditions, including pH and geometric isomerism, affect the structural chemistry.

In situsynchrotron powder X-ray diffraction measurements have demonstrated that the isostructural AUO4–x(A= alkaline earth metal cation) oxides CaUO4–xand α-Sr0.4Ca0.6UO4–xundergo a reversible phase transformation under reducing conditions at high temperatures associated with the ordering of in-plane oxygen vacancies resulting in the lowering of symmetry. When rhombohedral (space group R3̅m) CaUO4–xand α-Sr0.4Ca0.6UO4–xare heated to 450 and 400 °C, respectively, in a hydrogen atmosphere, they undergo a first-order phase transformation to a single phase structure which can be refined against a triclinic model in space group P1̅, δ-CaUO4–xand δ-Sr0.4Ca0.6UO4–x, where the oxygen vacancies are disordered initially. Continued heating results in the appearance of superlattice reflections, indicating the ordering of in-plane oxygen vacancies. Cooling ordered δ-CaUO4–xand δ-Sr0.4Ca0.6UO4–xto near room temperature results in the reformation of the disordered rhombohedral phases. Essential to the transformation is the generation of a critical amount of oxygen vacancies. Once these are formed, the transformation can be accessed continuously through thermal cycling, showing that the transformations are purely thermodynamic in origin. Stoichiometric structures of both oxides can be recovered by heating oxygen deficient CaUO4–xand α-Sr0.4Ca0.6UO4–xunder pure oxygen to high temperatures. When heated in air, the amount of oxygen vacancy defects that form in CaUO4–xand α-Sr0.4Ca0.6UO4–xare found to correlate with the Asite composition. The inclusion of the larger Sr2+cation on the Asite reduces defect–defect interactions, which increases the amount of defects that can form and lowers their formation temperature. The relative difference in the amount of defects that form can be understood on the basis of oxygen vacancy and U5+disordering as shown by both ab initiocalculations and estimated oxygen vacancy formation energies based on thermodynamic considerations. This difference in defect–defect interactions consequently introduces variations in the long-range ordered anionic lattice of the δ phases despite the isostructural relationship of the α structures of CaUO4–xand Sr0.4Ca0.6UO4–x. These results are discussed with respect to the influence the Asite cation has upon anion defect formation and ordering and are also compared to δ-SrUO4–x, the only other material known to be able to undergo a reversible symmetry lowering and disorder-to-order transformation with increasing temperature.

The first comprehensive structural study of the Ni–U–O system is reported. Single crystals of α-NiUO4, β-NiUO4, and NiU3O10were synthesized, and their structures were refined—using synchrotron single-crystal X-ray diffraction data supported by X-ray absorption spectroscopic measurements. α-NiUO4adopts an orthorhombic structure in space group Pbcnand is isostructural to CrUO4containing corrugated two-dimensional (2D) layers of corner-sharing UO6polyhedra and edge-sharing one-dimensional (1D) zigzag α-PbO2rutile-like chains of NiO6polyhedra in the [001] direction. β-NiUO4is isostructural to MgUO4and has an orthorhombic structure in space group Ibmm, which contains alternating 1D chains of edge-sharing UO6and NiO6polyhedra in the [001] direction as in regular TiO2rutile. NiU3O10forms a triclinic structure in space group P1̅ and is isostructural with CuU3O10, where it forms a three-dimensional (3D) framework structure built through a mixture of UO6and UO7polyhedra in which the NiO6polyhedra sit isolated within the framework. X-ray absorption near-edge structure (XANES) measurements, conducted using XANES mapping of single crystals, support the presence of hexavalent uranium in the three structures. The polymorphs of NiUO4were found to only form under high-pressure and high-temperature conditions (≥4 GPa and 700 °C). It is argued that this is a consequence of the relative size difference between the Ni2+and U6+cations, where the Ni2+cation is effectively too small for the Ibmmstructure and too large for the Pbcnstructure to form under ambient pressure conditions. This does not appear to be an issue for NiU3O10, which forms under ambient pressure conditions, where NiO6polyhedra sit isolated within the framework of 3D connected UO6/UO7polyhedra. Synthesis conditions indicate that β-NiUO4is the preferred higher-pressure phase and that the transformation to this occurs irreversibly at a temperature between 950 and 1000 °C, when P= 4 GPa. The routes toward the synthesis of the oxides and the associated structural and spectroscopic results are described with respect to the structural chemistry of the Ni–U–O system, the larger AUO4family of oxides (A = divalent or trivalent cation), and also their relation to the rutile-related family of oxides.

In situ synchrotron powder X-ray diffraction measurements have demonstrated that SrUO4undergoes a reversible phase transformation under reducing conditions at high temperatures, associated with the ordering of oxygen defects resulting in a lowering of crystallographic symmetry. When substoichiometric rhombohedral α-SrUO4–x, in space group R3̅mwith disordered in-plane oxygen defects, is heated above 200 °C in a hydrogen atmosphere it undergoes a first order phase transformation to a (disordered) triclinic polymorph, δ-SrUO4–x, in space group P1̅. Continued heating to above 450 °C results in the appearance of superlattice reflections, due to oxygen-vacancy ordering forming an ordered structure δ-SrUO4–x. Cooling δ-SrUO4–xtoward room temperature results in the reformation of the rhombohedral phase α-SrUO4–xwith disordered defects, confirming the reversibility of the transformation. This suggests that the transformation, resulting from oxygen vacancy ordering, is not a consequence of sample reduction or decomposition, but rather represents a change in the energetics of the system. A strong reducing atmosphere is required to generate a critical amount of oxygen defects in α-SrUO4–xto enable the transformation to δ-SrUO4–xbut once formed the transformation between these two phases can be induced by thermal cycling. The structure of δ-SrUO4–xat 1000 °C was determined using symmetry representation analysis, with the additional reflections indexed to a commensurate distortion vector k= ⟨1/4 1/4 3/4⟩. The ordered 2D layered triclinic structure of δ-SrUO4–xcan be considered a structural distortion of the disordered 2D layered rhombohedral α-SrUO4–xstructure through the preferential rearrangement of the in-plane oxygen vacancies. Ab initio calculations using density functional theory with self-consistently derived Hubbard U parameter support the assigned ordered defect superstructure model. Entropy changes associated with the temperature dependent short-range ordering of the reduced U species are believed to be important and these are discussed with respect to the results of the ab initio calculations.

Two novel alkali-metal uranyl borophosphates have been prepared and characterized for the first time, namely, K5(UO2)2[B2P3O12(OH)]2(OH)(H2O)2and K2(UO2)12[B(H2PO4)4](PO4)8(OH)(H2O)6denoted as KUPB1and KUPB2, respectively. KUPB1was obtained hydrothermally at 220 °C and crystallizes in a monoclinic structure in the chiral space group P21. The unit cell parameters of KUPB1are a= 6.7623(2) Å, b= 19.5584(7) Å, c= 11.0110(4) Å, α = γ = 90°, β = 95.579(3)°, and V= 1449.42(8) Å3. It features a unique three-dimensional (3D) open-framework structure, composed of two corner-sharing linked one-dimensional (1D) anionic borophosphates (BP), [B2P3O13]5–, along the aaxis and uranyl phosphate (UP), [(UO2)(PO4)3]7–, chains along the caxis, further bridged by PO4tetrahedra. Multi-intersectional channels can be observed within the structure, in which the largest 11-ring (11-R) tunnel size is ∼7.0 Å × 8.8 Å. Its simplified framework can be described as a new 4-nodal net topological type with a point symbol of {4.84.10}{42.6}2{43.62.83.102}{82.10}. By modification of the synthetic conditions of KUPB1through an increase in the amount of H3BO3as flux 4-fold and a reduction of water as the reaction medium, the novel compound KUPB2is generated. The unit cell parameters of KUPB2are a= b= 21.8747(3) Å, c= 7.0652(2) Å, α = β = γ = 90°, and V= 3380.72(12) Å3. KUPB2crystallizes in a tetragonal structure in the polar space group I4̅2m, and its structure is based on a highly complex 3D framework, {(UO2)12[B(PO4)4](PO4)8}9–, in which 1D 8-R UP [(UO2)(PO4)]−tubes can be observed along the caxis. The [(UO2)(PO4)]−tubes consist of three uranyl chains along the caxis, which are linked alternately by [PO4]3–tetrahedra. Those isolated 1D [(UO2)(PO4)]−tubes are further bridged through [(UO2)4B(PO4)4]−clusters, forming an exceptional 3D open-framework structure. Its simplified cation network is a new 5-nodal net topological type such as {32.43.5.62.7.8}8{34.45.54.62}8{4.62.83}4{42.6}4{44.62}. Their facile hydrothermal synthetic routes, porous structure topology, thermal stability, and Raman spectroscopy properties are reported and discussed.

In situ neutron and synchrotron X-ray diffraction studies demonstrate that SrUO4acts as an oxygen transfer agent, forming oxygen vacancies under both oxidizing and reducing conditions. Two polymorphs of SrUO4are stable at room temperature, and the transformation between these is observed to be associated with thermally regulated diffusion of oxygen ions, with partial reduction of the U6+playing a role in both the formation of oxygen deficient α-SrUO4−δand its subsequent transformation to stoichiometric β-SrUO4. This is supported by ab initio calculations using density functional theory calculations. The oxygen vacancies play a critical role in the first order transition that SrUO4undergoes near 830 °C. The changes in the oxidation states and U geometry associated with the structural phase transition have been characterized using X-ray absorption spectroscopy, synchrotron X-ray diffraction, and neutron diffraction.

A systematic investigation examining the origins of structural distortions in rutile-related ternary uranium AUO4oxides using a combination of high-resolution structural and spectroscopic measurements supported by ab initiocalculations is presented. The structures of β-CdUO4, MnUO4, CoUO4, and MgUO4are determined at high precision by using a combination of neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (S-XRD) or single crystal X-ray diffraction. The structure of β-CdUO4is best described by space group Cmmmwhereas MnUO4, CoUO4, and MgUO4are described by the lower symmetry Ibmmspace group and are isostructural with the previously reported β-NiUO4[Murphy et al. Inorg. Chem.2018, 57, 13847]. X-ray absorption spectroscopy (XAS) analysis shows all five oxides contain hexavalent uranium. The difference in space group can be understood on the basis of size mismatch between the A2+and U6+cations whereby unsatisfactory matching results in structural distortions manifested through tilting of the AO6polyhedra, leading to a change in symmetry from Cmmmto Ibmm. Such tilts are absent in the Cmmmstructure. Heating the Ibmm AUO4oxides results in reduction of the tilt angle. This is demonstrated for MnUO4where in situS-XRD measurements reveal a second-order phase transition to Cmmmnear T= 200 °C. Based on the extrapolation of variable temperature in situS-XRD data, CoUO4is predicted to undergo a continuous phase transition to Cmmmat ∼1475 °C. Comparison of the measured and computed data highlights inadequacies in the DFT+Uapproach, and the conducted analysis should guide future improvements in computational methods. The results of this investigation are discussed in the context of the wider AUO4family of oxides.