Probing the Long- and Short-Range Structural Chemistry in the C-Type Bixbyite Oxides Th0.40Nd0.48Ce0.12O1.76, Th0.47Nd0.43Ce0.10O1.785, and Th0.45Nd0.37Ce0.18O1.815 via Synchrotron X-ray Diffraction and Absorption Spectroscopy.

The long- and short-range structural chemistry of the C-type bixbyite compounds Th0.40Nd0.48Ce0.12O1.76, Th0.47Nd0.43Ce0.10O1.785, and Th0.45Nd0.37Ce0.18O1.815 is systematically examined using synchrotron X-ray powder diffraction (S-PXRD), high-energy resolution fluorescence detection X-ray absorption near edge (HERFD-XANES), and extended X-ray absorption fine structure spectroscopy (EXAFS) measurements supported by electronic structure calculations. S-PXRD measurements revealed that the title compounds all form classical C-type bixbyite structures in space group Ia3̅ that have disordered cationic crystallographic sites with further observation of characteristic superlattice reflections corresponding to oxygen vacancies. Despite the occurrence of oxygen vacancies, HERFD-XANES measurements on the Ce L3-edge revealed that Ce incorporates as Ce4+ into the structures but involves local distortion that resembles cluster behavior and loss of nearest-neighbors. In comparison, HERFD-XANES measurements on the Nd L3-edge supported by electronic structure calculations reveal that Nd3+ adopts a local coordination environment similar to the long-range C-type structure while providing charge balancing for the formation of oxygen defects. Th L3-edge EXAFS analysis reveals shorter average Th-O distances in the title compounds in comparison to pristine ThO2 in addition to shorter Th-O and Th-Ce distances compared to Th-Th or Ce-Ce in the corresponding F-type binary oxides (ThO2 and CeO2). These distances are further found to decrease with the increased Nd content of the structures despite simultaneous observation of the overall lattice structure progressively expanding. Linear combination calculations of the M-O bond lengths are used to help explain these observations, where the role of oxygen defects, via Nd3+ incorporation, induces local bond contraction and enhanced Th cation valence, leading to the observed increased lattice expansion with progressive Nd3+ incorporation. Overall, the investigation points to the significance of dissimilar cations exhibiting variable short-range chemical behavior and how it can affect the long-range structural chemistry of complex oxides.

Deconvoluting Cr states in Cr-doped UO2 nuclear fuels via bulk and single crystal spectroscopic studies.

Cr-doped UO2 is a leading accident tolerant nuclear fuel where the complexity of Cr chemical states in the bulk material has prevented acquisition of an unequivocal understanding of the redox chemistry and mechanism for incorporation of Cr in the UO2 matrix. To resolve this, we have used electron paramagnetic resonance, high energy resolution fluorescence detection X-ray absorption near energy structure and extended X-ray absorption fine structure spectroscopic measurements to examine Cr-doped UO2 single crystal grains and bulk material. Ambient condition measurements of the single crystal grains, which have been mechanically extracted from bulk material, indicated Cr is incorporated substitutionally for U+4 in the fluorite lattice as Cr+3 with formation of additional oxygen vacancies. Bulk material measurements reveal the complexity of Cr states, where metallic Cr (Cr0) and oxide related Cr+2 and Cr+32O3 were identified and attributed to grain boundary species and precipitates, with concurrent (Cr+3xU+41-x)O2-0.5x lattice matrix incorporation. The deconvolution of chemical states via crystal vs. powder measurements enables the understanding of discrepancies in literature whilst providing valuable direction for safe continued use of Cr-doped UO2 fuels for nuclear energy generation.

Understanding the role of flux, pressure and temperature on polymorphism in ThB2O5.

A novel polymorph of ThB2O5, denoted as ß-ThB2O5, was synthesised under high-temperature high-pressure (HT/HP) conditions. Via single crystal X-ray diffraction measurements, ß-ThB2O5 was found to form a three-dimensional (3D) framework structure where thorium atoms are ten-fold oxygen coordinated forming tetra-capped trigonal prisms. The only other known polymorph of ThB2O5, denoted α, synthesised herein using a known borax, B2O3-Na2B4O7, high temperature solid method, was found to transform to the ß polymorph when exposed to conditions of 4 GPa and ∼900 °C. Compared to the α polymorph, ß-ThB2O5 has smaller molar volume by approximately 12%. Exposing a mixture of the α and ß polymorphs to HT/HP conditions ex situ further demonstrated the preferred higher-pressure phase being ß, with no α phase material being observed via Rietveld refinements against laboratory X-ray powder diffraction (PXRD) measurements. In situ heating PXRD measurements on α-ThB2O5 from RT to 1030 °C indicated that α-ThB2O5 transforms to the ß variant at approximately 900 °C via a 1st order mechanism. ß-ThB2O5 was found to exist only over a narrow temperature range, decomposing above 1050 °C. Ab initio calculations using density functional theory (DFT) with the Hubbard U parameter indicated, consistent with experimental observations, that ß is both the preferred phase at higher temperatures and high pressures. Interestingly, it was found by switching from B2O3-Na2B4O7 to H3BO3-Li2CO3 flux using consistent high temperature solid state conditions for the synthesis of the α variant, ß-ThB2O5 could be generated. Comparison of their single crystal measurements showed this was identical to that obtained from HT/HP conditions.

Advances and perspectives of actinide chemistry from ex situ high pressure and high temperature chemical studies.

High pressure high temperature (HP/HT) studies of actinide compounds allow the chemistry and bonding of among the most exotic elements in the periodic table to be examined under the conditions often only found in the severest environments of nature. Peering into this realm of physical extremity, chemists have extracted detailed knowledge of the fundamental chemistry of actinide elements and how they contribute to bonding, structure formation and intricate properties in compounds under such conditions. The last decade has resulted in some of the most significant contributions to actinide chemical science and this holds true for ex situ chemical studies of actinides resulting from HP/HT conditions of over 1 GPa and elevated temperature. Often conducted in tandem with ab initio calculations, HP/HT studies of actinides have further helped guide and develop theoretical modelling approaches and uncovered associated difficulties. Accordingly, this perspective article is devoted to reviewing the latest advancements made in actinide HP/HT ex situ chemical studies over the last decade, the state-of-the-art, challenges and discussing potential future directions of the science. The discussion is given with emphasis on thorium and uranium compounds due to the prevalence of their investigation but also highlights some of the latest advancements in high pressure chemical studies of transuranium compounds. The perspective also describes technical aspects involved in HP/HT investigation of actinide compounds.

Incorporation of iodine into uranium oxyhydroxide phases.

Herein, we have synthesised a novel uranium oxyhydroxide (UOH) phase, Rb2K2[(UO2)6O4(OH)6]·(IO3)2, under hydrothermal conditions which intercalates IO3-via a hybrid salt-inclusion and host-guest mechanism. The mechanism is based on favorable intermolecular bonding between disordered Rb+/K+ and IO3- ions and hydroxyl and layer void positions respectively. To examine whether the intercalation may occur ubiquitously for UOH phases, the known UOH mineral phases metaschoepite ([(UO2)8O2(OH)12]·12H2O), compreignacite (K2[(UO2)6O4(OH)6]·7H2O) and also related ß-UO2(OH)2 were synthesised and exposed to aqueous I- and IO3- for 1 month statically at RT and 60 °C in air and the solid analysed using laser ablation inductively coupled plasma mass spectroscopy. Measurements indicate intercalation can occur homogeneously, but the affinity is dependent upon the structure of the UOH phases and temperature, where higher temperatures and when the interlayer space is free of initial moieties are favoured. It was also found that after repeated washing of the UOH samples with DI water the intercalated iodine was retained. UOH phases are known to form during the oxidative corrosion of spent nuclear fuel during an accident scenario in the near field, this work suggests they may help retard the transport of radiolytic iodine into the environment during a long-term release event.

Chromium Doped UO2-Based Ceramics: Synthesis and Characterization of Model Materials for Modern Nuclear Fuels.

Cr-doped UO2 as a modern nuclear fuel type has been demonstrated to increase the in-reactor fuel performance compared to conventional nuclear fuels. Little is known about the long-term stability of spent Cr-doped UO2 nuclear fuels in a deep geological disposal facility. The investigation of suitable model materials in a step wise bottom-up approach can provide insights into the corrosion behavior of spent Cr-doped nuclear fuels. Here, we present new wet chemical approaches providing the basis for such model systems, namely co-precipitation and wet coating. Both were successfully tested and optimized, based on detailed analyses of all synthesis steps and parameters: Cr-doping method, thermal treatment, reduction of U3O8 to UO2, green body production, and pellet sintering. Both methods enable the production of suitable model systems with a similar microstructure and density as a reference sample from AREVA. In comparison with results from the classical powder route, similar trends upon grain size and lattice parameter were determined. The results of this investigation highlight the significance of subtly different synthesis routes on the properties of Cr-doped UO2 ceramics. They enable a reproducible tailor-made well-defined microstructure, a homogeneous doping, for example, with lanthanides or alpha sources, the introduction of metallic particles, and a dust-free preparation.

Tilting and Distortion in Rutile-Related Mixed Metal Ternary Uranium Oxides: A Structural, Spectroscopic, and Theoretical Investigation.

A systematic investigation examining the origins of structural distortions in rutile-related ternary uranium AUO4 oxides using a combination of high-resolution structural and spectroscopic measurements supported by ab initio calculations is presented. The structures of ß-CdUO4, MnUO4, CoUO4, and MgUO4 are 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 ß-CdUO4 is best described by space group Cmmm whereas MnUO4, CoUO4, and MgUO4 are described by the lower symmetry Ibmm space group and are isostructural with the previously reported ß-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 AO6 polyhedra, leading to a change in symmetry from Cmmm to Ibmm. Such tilts are absent in the Cmmm structure. Heating the Ibmm AUO4 oxides results in reduction of the tilt angle. This is demonstrated for MnUO4 where in situ S-XRD measurements reveal a second-order phase transition to Cmmm near T = 200 °C. Based on the extrapolation of variable temperature in situ S-XRD data, CoUO4 is predicted to undergo a continuous phase transition to Cmmm at ∼1475 °C. Comparison of the measured and computed data highlights inadequacies in the DFT+U approach, and the conducted analysis should guide future improvements in computational methods. The results of this investigation are discussed in the context of the wider AUO4 family of oxides.

The first actinide polyiodate: a complex multifunctional compound with promising X-ray luminescence properties and proton conductivity.

Herein we report the first example of an actinide polyiodate compound, namely K4[(UO2)2(IO3)6(I4O11)]·(HIO3)4(H2O)6 (UPI-1), which was obtained from slow evaporation of nitric acid with a high I/U ratio. Spectroscopic measurements indicate that UPI-1 possesses X-ray luminescence properties applicable for X-ray scintillation and also exhibits modest protonic conduction.

Extreme condition high temperature and high pressure studies of the K-U-Mo-O system.

Herein the first examples of alkali earth uranyl molybdates synthesised using extreme conditions of high temperature and high pressure (HT/HP) methods, namely K2[UO2(Mo2O7)2], K2[(UO2)2(Mo(vi)4Mo(iv)(OH)2)O16], K3[(UO2)6(OH)2(MoO4)6(MoO3OH)] and K5[(UO2)10MoO5O11OH]·H2O, are described and characterised. K2[UO2(Mo2O7)2] forms a monoclinic 2D layered structure in space group P21/c that consists of interlinking Mo2O7 dimers that link isolated UO22+ moieties forming [UO2(Mo2O7)2]2- layers which are separated by K+ cations. K2[(UO2)2(Mo(vi)4Mo(iv)(OH)2)O16] forms a disordered triclinic 3D framework structure in space group P1[combining macron]. The structure consists of isolated UO22+ moieties connected in a layered configuration via Mo(vi)O6 polyhedra of which the layers are bridged by Mo(iv)O6 polyhedra that are partially positionally disordered by charge balancing K+ and bridging Mo4+ cations. K3[(UO2)6(OH)2(MoO4)6(MoO3OH)] adopts a disordered orthorhombic 3D framework structure in space group Pbcm consisting of small channels and large cavities built upon corner sharing MoO4 and UO22+ moieties that respectively encapsulate ordered and disordered K+ cations. K5[(UO2)10MoO5O11OH]·H2O forms a triclinic 3D framework structure in space group P1[combining macron] consisting of interlinking UO6, UO7 and MoO5 polyhedra which utilise cation-cation interactions between UO22+ moieties to create infinite channels parallel to the [001] direction which contain partially disordered K+ cations and H2O molecules. A combination of single crystal X-ray diffraction, bond valence sums calculations and scanning electron microscopy with energy dispersive X-ray spectroscopic measurements was used to characterise all obtained samples in this investigation. The structures uncovered in this investigation are discussed systematically in detail with other members of the broader A+-U-Mo-O system from the literature where the relationship between the degree of pressure applied and U/Mo ratio used during synthesis on the ability to obtain high dimensional structures via condensation and oligomerization of polyhedra is identified and discussed in detail.

Insights into the Structural Chemistry of Anhydrous and Hydrous Hexavalent Uranium and Neptunium Dinitrato, Trinitrato, and Tetranitrato Complexes.

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 Pbca consisting of antiparallel aligned [UO2(NO3)3]- moieties. K2[UO2-cis-(NO3)4] adopts a monoclinic structure in space group P21/c consisting of [UO2(NO3)4]2- moieties with two monodentate and two bidentate nitrate ligands that are arranged in a cis configuration about the uranyl, UO22+, center. Previous investigations have only identified trans variants of this monoclinic structure, and this is the first report of the cis form and also the occurrence of geometric isomerism in uranyl nitrates. [NpO2(NO3)2(H2O)2]·4H2O adopts an orthorhombic structure in space group Cmc21 consisting of parallel aligned [NpO2(NO3)2(H2O)2] moieties that are in a trans configuration with respect to the bidentate nitrate ligands. Cs[NpO2(NO3)3] adopts a hexagonal structure in space group R3c consisting 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.

Controlling Oxygen Defect Formation and Its Effect on Reversible Symmetry Lowering and Disorder-to-Order Phase Transformations in Nonstoichiometric Ternary Uranium Oxides.

In situ synchrotron powder X-ray diffraction measurements have demonstrated that the isostructural AUO4- x ( A = alkaline earth metal cation) oxides CaUO4- x and α-Sr0.4Ca0.6UO4- x undergo 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 R3̅ m) CaUO4- x and α-Sr0.4Ca0.6UO4- x are 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- x and 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- x and δ-Sr0.4Ca0.6UO4- x to 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- x and α-Sr0.4Ca0.6UO4- x under pure oxygen to high temperatures. When heated in air, the amount of oxygen vacancy defects that form in CaUO4- x and α-Sr0.4Ca0.6UO4- x are found to correlate with the A site composition. The inclusion of the larger Sr2+ cation on the A site 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 initio calculations 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- x and Sr0.4Ca0.6UO4- x. These results are discussed with respect to the influence the A site 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.

High-Pressure Synthesis, Structural, and Spectroscopic Studies of the Ni-U-O System.

The first comprehensive structural study of the Ni-U-O system is reported. Single crystals of α-NiUO4, ß-NiUO4, and NiU3O10 were synthesized, and their structures were refined-using synchrotron single-crystal X-ray diffraction data supported by X-ray absorption spectroscopic measurements. α-NiUO4 adopts an orthorhombic structure in space group Pbcn and is isostructural to CrUO4 containing corrugated two-dimensional (2D) layers of corner-sharing UO6 polyhedra and edge-sharing one-dimensional (1D) zigzag α-PbO2 rutile-like chains of NiO6 polyhedra in the [001] direction. ß-NiUO4 is isostructural to MgUO4 and has an orthorhombic structure in space group Ibmm, which contains alternating 1D chains of edge-sharing UO6 and NiO6 polyhedra in the [001] direction as in regular TiO2 rutile. NiU3O10 forms 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 UO6 and UO7 polyhedra in which the NiO6 polyhedra 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 NiUO4 were 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 Ibmm structure and too large for the Pbcn structure to form under ambient pressure conditions. This does not appear to be an issue for NiU3O10, which forms under ambient pressure conditions, where NiO6 polyhedra sit isolated within the framework of 3D connected UO6/UO7 polyhedra. Synthesis conditions indicate that ß-NiUO4 is 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 AUO4 family of oxides (A = divalent or trivalent cation), and also their relation to the rutile-related family of oxides.

Unexpected Crystallographic Phase Transformation in Nonstoichiometric SrUO4- x: Reversible Oxygen Defect Ordering and Symmetry Lowering with Increasing Temperature.

In situ synchrotron powder X-ray diffraction measurements have demonstrated that SrUO4 undergoes 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 R3̅ m with 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- x toward room temperature results in the reformation of the rhombohedral phase α-SrUO4- x with 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- x to enable the transformation to δ-SrUO4- x but once formed the transformation between these two phases can be induced by thermal cycling. The structure of δ-SrUO4- x at 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- x can be considered a structural distortion of the disordered 2D layered rhombohedral α-SrUO4- x structure 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.

Porous Uranyl Borophosphates with Unique Three-Dimensional Open-Framework Structures.

Two novel alkali-metal uranyl borophosphates have been prepared and characterized for the first time, namely, K5(UO2)2[B2P3O12(OH)]2(OH)(H2O)2 and K2(UO2)12[B(H2PO4)4](PO4)8(OH)(H2O)6 denoted as KUPB1 and KUPB2, respectively. KUPB1 was obtained hydrothermally at 220 °C and crystallizes in a monoclinic structure in the chiral space group P21. The unit cell parameters of KUPB1 are a = 6.7623(2) Å, b = 19.5584(7) Å, c = 11.0110(4) Å, α = γ = 90°, ß = 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 a axis and uranyl phosphate (UP), [(UO2)(PO4)3]7-, chains along the c axis, further bridged by PO4 tetrahedra. 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 KUPB1 through an increase in the amount of H3BO3 as flux 4-fold and a reduction of water as the reaction medium, the novel compound KUPB2 is generated. The unit cell parameters of KUPB2 are a = b = 21.8747(3) Å, c = 7.0652(2) Å, α = ß = γ = 90°, and V = 3380.72(12) Å3. KUPB2 crystallizes 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 c axis. The [(UO2)(PO4)]- tubes consist of three uranyl chains along the c axis, 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.

Nonstoichiometry in Strontium Uranium Oxide: Understanding the Rhombohedral-Orthorhombic Transition in SrUO4.

In situ neutron and synchrotron X-ray diffraction studies demonstrate that SrUO4 acts as an oxygen transfer agent, forming oxygen vacancies under both oxidizing and reducing conditions. Two polymorphs of SrUO4 are 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 U(6+) playing a role in both the formation of oxygen deficient α-SrUO4-δ and its subsequent transformation to stoichiometric ß-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 SrUO4 undergoes 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.