Controlling Oxygen Defect Formation and Its Effect on Reversible Symmetry Lowering and Disorder-to-Order Phase...

In situ synchrotron powder X-ray diffraction measurements have demonstrated that the isostructural AUO_{4- x} ( A = alkaline earth metal cation) oxides CaUO_{4- x} and α-Sr_0.4Ca_0.6UO_{4- 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 R3̅ m) CaUO_{4- x} and α-Sr_0.4Ca_0.6UO_{4- 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-CaUO_{4- x} and δ-Sr_0.4Ca_0.6UO_{4- 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-CaUO_{4- x} and δ-Sr_0.4Ca_0.6UO_{4- 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 CaUO_{4- x} and α-Sr_0.4Ca_0.6UO_{4- x} under pure oxygen to high temperatures. When heated in air, the amount of oxygen vacancy defects that form in CaUO_{4- x} and α-Sr_0.4Ca_0.6UO_{4- 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 CaUO_{4- x} and Sr_0.4Ca_0.6UO_{4- 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-SrUO_{4- x}, the only other material known to be able to undergo a reversible symmetry lowering and disorder-to-order transformation with increasing temperature.