Unexpected Crystallographic Phase Transformation in Nonstoichiometric SrUO_4–<i>x</i>: 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-SrUO_4–<i>x</i>, in space group <i>R</i>3̅<i>m</i> 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-SrUO_4–<i>x</i>, in space group <i>P</i>1̅. Continued heating to above 450 °C results in the appearance of superlattice reflections, due to oxygen-vacancy ordering forming an ordered structure δ-SrUO_4–<i>x</i>. Cooling δ-SrUO_4–<i>x</i> toward room temperature results in the reformation of the rhombohedral phase α-SrUO_4–<i>x</i> 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-SrUO_4–<i>x</i> to enable the transformation to δ-SrUO_4–<i>x</i> but once formed the transformation between these two phases can be induced by thermal cycling. The structure of δ-SrUO_4–<i>x</i> at 1000 °C was determined using symmetry representation analysis, with the additional reflections indexed to a commensurate distortion vector <b>k</b> = ⟨1/4 1/4 3/4⟩. The ordered 2D layered triclinic structure of δ-SrUO_4–<i>x</i> can be considered a structural distortion of the disordered 2D layered rhombohedral α-SrUO_4–<i>x</i> 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