Pressure induced reduction in SrUO4 – A topotactic pathway to accessing extreme incompressibility

A combined experimental and theoretical investigation is presented which examines the compressibility of two SrUO _4-x polymorphs, α and β, under hydrostatic conditions and explains the contrasting chemical and mechanical behaviours of these in terms of differences in oxygen defect formation chemistry. Via Rietveld refinements against in situ neutron powder diffraction data, the uranyl bonds in α-SrUO ₄ and β-SrUO ₄ are shown to expand by approximately 20% when subjected to hydrostatic pressures of up to 6 GPa. This is consistent with a reduction of the uranium formal oxidation state from +6 towards +4. Supported by ab initio calculations using density functional theory, the origin of the reduction is ascribed to the development of oxygen vacancies leading to the formation of SrUO _4-x phases. Remarkably, very different apparent bulk moduli of 591(100) and 60(3) GPa were found for α-SrUO _4-x and β-SrUO _4-x respectively, which are attributed to the difference in the preferred lattice sites for oxygen defect formation. In α-SrUO _4-x oxygen vacancies preferentially form at the in-plane equatorial sites of the UO ₈ polyhedra rather than the uranyl sites leading to enhanced electrostatic interlayer repulsion with increasing pressure. In contrast, the oxygen vacancies in β-SrUO _4-x form at the uranyl sites of the UO ₆ polyhedra, which negates the electrostatic repulsion and therefore it is more easily compressed.