High-pressure evolution of novel R2CuTiO6 (R = Tb – Lu) perovskite phases

Perovskite oxides of the R2CuTiO6 type with a trivalent rare earth element (R) at the A-cation site and co-occupancy of the B-cation site with the Jahn-Teller d9 Cu2+ and d0 Ti4+ transition metals, provides an interesting playground for various chemical and physical phenomena. However, conventional solid-state synthesis under ambient-pressure (AP) yields the perovskite structure only for the largest R constituents from La to Gd; with the smaller rare earth elements (Y, Tb–Lu) a non-perovskite (hexagonal BaTiO3-type) structure is formed. Here we demonstrate that through high-pressure (HP) high-temperature treatment, the hexagonal AP structure can be converted into a distorted (orthorhombic) perovskite structure for all the smaller R constituents. The critical pressure needed for the conversion increases with decreasing R3+ ion size, up to ca. 6 GPa for R = Lu. Moreover, a novel intermediate phase is found to form for most of the R constituents when pressures lower than the critical pressure are applied. We have employed both X-ray diffraction and UV–vis spectroscopy analyses to systematically follow the phase formation schemes for the different R constituents.