Browsing by Author "Beridze, G"

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    Controlling oxygen defect formation and its effect on reversible symmetry lowering and disorder-to-order phase transformations in nonstoichiometric ternary uranium oxides
    (American Chemical Society, 2019-04-09) Murphy, GL; Wang, CH; Zhang, Z; Kowalski, PM; Beridze, G; Avdeev, M; Muránsky, O; Brand, HEA; Gu, QF; Kennedy, BJ
    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 R3̅m) 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. © 2019 American Chemical Society
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    Ion-transport phenomena and anomalous transformations in strontium uranium oxides.
    (International Union of Crystallography, 2017-12-01) Murphy, GL; Zhang, Z; Avdeev, M; Wang, CH; Beridze, G; Kowalski, PM; Gu, QF; Kimpton, JA; Johannessen, B; Kennedy, BJ
    Structural-chemical elucidation of low dimensional ternary uranium oxide systems is considered an essential aspect of thenuclear fuel cycle since understanding of their physicochemical properties may guide the storage and disposal of spentnuclear fuel [1]. The study of these systems allows for further exploration of the peculiar, exotic and poorly knownproperties of materials containing, or which can access, 5f electrons. SrUO₄ exemplifies this, a potential waste form resultingfrom reaction between spent UO₂+x fuel and the fission daughter Sr-90. We have found, through a combination of in situsynchrotron X-ray powder diffraction and X-ray absorption spectroscopy, that during its first order rhombohedral-orthorhombic transition under oxidising conditions, the rhombohedral form of SrUO₄, α, undergoes a spontaneousreduction of the uranium valence state through oxygen vacancy formation [2]. The process is synergetic, as the triality ofoxygen vacancy formation, subsequent ion diffusion and uranium reduction, seemingly reduces the activation energy barrierfor the transformation to the thermodynamically favoured stoichiometric orthorhombic form, β-SrUO₄. However formation ofthe orthorhombic form is only possible if a source of oxygen is present, without this, the oxygen deficient α-SrUO₄-xremains rhombohedral as shown by in situ neutron powder diffraction measurements. These experimental observations arefurther supported by ab initio DFT+U calculations using the self consistently calculated Hubbard U parameter values andbond valence sums calculations [2-3]. These methods indicate the affinity for α-SrUO₄-x to retain oxygen vacancies asopposed to β-SrUO₄, a consequence of the crystal lattice’s ability to stabilise the coordination environment of the Sr²⁺ cationvia the flexibility of uranium to undergo reduction through vacancy formation. CaUO4, isostructural to α-SrUO₄ , but unlike α-SrUO₄ does not have a stable orthorhombic polymorph as shown by both insitu synchrotron X-ray powder diffraction measurements and ab initio calculations. Introducing Sr ions into the CaUO₄ latticein the form of a solid solution, α-Sr₁-xCaxUO₄ (0 < x < 0.4), provides a means to atomically engineer the lattice to promoteoxygen vacancy formation, and presumably diffusion, at high temperatures. When CaUO₄ or α-SrUO₄ is treated underhighly reducing conditions, both materials undergo unusual reconstructive phase transformations at high temperatures to amonoclinic structure. These phase transformations are reversible, and cooling the sample yields the correspondingrhombohedral structure again. It is remarkable that the ordered monoclinic structure is favoured at high temperatures andthe disordered rhombohedral structure at low temperatures. This investigation in SrUO₄ highlights the rich and remarkablestructural chemistry and crystallography that may be found within poorly understood actinide systems whilst demonstratingthe successful marriage of experimental and theoretical approaches towards elucidating their chemical and physicalphenomena. © International Union of Crystallography
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    Nonstoichiometry in strontium uranium oxide: understanding the rhombohedral–orthorhombic transition in SrUO4
    (American Chemical Society, 2016-08-29) Murphy, GL; Kennedy, BJ; Kimpton, JA; Gu, QF; Johannessen, B; Beridze, G; Kowalski, PM; Bosbach, D; Avdeev, M; Zhang, Z
    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 U6+ playing a role in both the formation of oxygen deficient α-SrUO4−δ and 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 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. © 2016 American Chemical Society
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    Unexpected crystallographic phase transformation in nonstoichiometric SrUO4–x: Reversible oxygen defect ordering and symmetry lowering with increasing temperature
    (American Chemical Society, 2018-05-01) Murphy, GL; Wang, CH; Beridze, G; Zhang, Z; Kimpton, JA; Avdeev, M; Kowalski, PM; Kennedy, BJ
    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 R3̅m 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. © 2018 American Chemical Society
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    The unusual structural chemistry of uranium: controlling phase transformations in ternary uranium oxides
    (Society of Crystallographers in Australia and New Zealand, 2017-12-03) Murphy, GL; Wang, CH; Beridze, G; Zhang, Z; Avdeev, M; Kowalski, PM; Brand, HEA; Johannessen, B; Kennedy, BJ
    Structural investigations of ternary uranium oxides are pertinent to the management of spent nuclear fuel since such environmentally hazardous and potentially dangerous secondary phases may form during the process and/or storage of the material [1]. They further allow for the exploration of the peculiar, exotic and poorly known properties of materials containing, or which can access 5f electrons. The rhombohedral oxides AUO4 for A = α-Sr or Ca in space group 𝑅𝑅3�𝑚𝑚 exemplifies this. We have found through a combination of in situ synchrotron X-ray powder diffraction and X-ray absorption spectroscopy, that α-SrUO4 undergoes a fascinating first-order phase transformation under oxidising conditions [2]. This involves the synergetic loss of lattice oxygen resulting in oxygen vacancy defect formation and reduction of the uranium cations, which seemingly reduces the activation energy barrier for transformation to its orthorhombic form, β-SrUO4. Under similar conditions, CaUO4 does not display any transformative behaviour, however, defects can be engineered through the substitution of Ca2+ for Sr2+ in the solid solution α-SrxCa1-xUO4 when heated to high temperature under oxidising conditions. The introduction of Sr2+ cations in α-SrxCa1-xUO4 was found to decrease the temperature at which oxygen vacancy defects form. This phenomenon was rationalised as a consequence of the introduction of Sr2+ cations leading to lattice expansion, which causes the proximity of defects to increase. This subsequently reduces free energy increasing defect-defect interactions, allowing defects to form at lower temperature. Remarkably, when heated under reducing conditions, the disordered oxygen defect containing rhombohedral α-SrUO4-x structure undergoes a reversible first-order phase transformation that involves the ordering of the oxygen defects resulting in lowering of the crystallographic symmetry to triclinic in space group P 1� denoted δ-SrUO4-x. This remarkable transformation, which implies entropy is being decreased as temperature increases, could be replicated in CaUO4 and also in α-Sr0.4Ca0.6UO4 where the transformation temperature is reduced by increasing the Sr2+ content, consistent with the effects of reducing defect-defect interactions. The S-XRD data shows the structure of δ-CaUO4-x to be incommensurate whereas δ-SrUO4-x is commensurate. This implies a miscibility gap may exist between the isostructural CaUO4 and α-SrUO4 related to shortrange order. This investigation demonstrates the rich and fascinating crystal chemistry present in uranium oxides, which, in some cases, may have profound societal importance if it can either be safely used or if associated properties can be replicated into non-actinide materials.