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 U⁶⁺ playing a role in both the formation of oxygen deficient α-SrUO_4−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