On the Complex Structural Picture of the Ionic Conductor Sr₆Ta₂O₁₁

Abstract

Sr 6 Ta 2 O 11 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 Sr 6 Ta 2 O 11 in its dry and hydrated state, based on DFT calculations and NPD measurements. At low temperatures, dry Sr 6 Ta 2 O 11 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 Sr 6 Ta 2 O 10 (OH) 2 was determined as monoclinic with significant distortions of the anion and metal cation sites.

Selective Interstitial Hydration Explains Anomalous Structural Distortions and Ionic Conductivity in 6H-Ba4Ta2O9·1/2H2O

“…However, in all such cases, the lattice parameter perpendicular to the 2-D intercalation layers undergoes a large first-order expansion upon hydration. Hydration in close-packed oxides also commonly occurs via hydroxylation, ,− but this generally requires pre-existing oxygen vacancies, such as in the proton conductor Ba 3 Ca 1.18 Ta 1.82 O 8.73 . The most similar case to 6H-Ba 4 Ta 2 O 9 may be the zircon-type phase stetindite, CeSiO 4 , in which H 2 O is proposed to be confined in 1-D channels narrow enough to modify the lattice expansion behavior from 200 to 700 °C.…”