Ion-transport phenomena and anomalous transformations in strontium uranium oxides.

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

Keywords

Uranium, Spectroscopy, Crystallography, X-ray diffraction, Phase transformations, Neutron diffraction

Citation

Murphy, G. L., Zhang, Z., Avdeev, M., Wang, C.-H., Beridze, G., Kowalski, P., Gu, Q., Kimpton, J., Johannessen, B., & Kennedy, B. (2017, January). Ion-transport phenomena and anomalous transformations in strontium uranium oxides. Paper presented at the XXIV IUCr Congress Hyderabad, India, 21-28 August 2017. In Acta Crystallographica Section A, 73(a2), C1428. doi:10.1107/S2053273317081487