On the Complex Structural Picture of the Ionic Conductor Sr6Ta2O11

Sr6Ta2O11 presents an interesting model system of a highly defective and disordered complex perovskite, exhibiting oxide ion conductivity at high temperatures as well as proton conductivity when hydrated by presence of water vapor. In this paper, we present a comprehensive structural study of Sr6Ta2O11 in its dry and hydrated state, based on DFT calculations and NPD measurements. At low temperatures, dry Sr6Ta2O11 has a tetragonal symmetry with ordered oxygen vacancies. The oxygen vacancy induces perturbations to its near surroundings, disturbing particularly the A site cations. As a consequence, the elaborate structural picture collapses at higher temperatures. The high-temperature structure has a cubic symmetry with fractionally occupied structural oxygen sites and disorder on the oxygen and strontium sublattices. The structure of the fully hydrated compound Sr6Ta2O10(OH)2 was determined as monoclinic with significant distortions of the anion and metal cation sites.