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

Abstract

The mixed ionic−electronic conductor 6H-Ba 4 Ta 2 O 9 undergoes an unconventional symmetry-lowering lattice distortion when cooled below 1100 K in the presence of atmospheric water. This temperature corresponds to the onset of hydration, which reaches a maximum value for 6H-Ba 4 Ta 2 O 9 •1/ 2H 2 O below ∼500 K. We use a combination of diffraction, ab initio calculations, and spectroscopy to show that both processes are intimately linked. The presence of very large Ba 2+ cations in octahedral interstitial sites (B sites of its hexagonal perovskite-type structure) forces the adjacent vacant octahedral interstitial sites also to expand, making room for them to incorporate hydration species with a total stoichiometric H 2 O in constrained and highly acidic environments, where they show structural and dynamic characteristics intermediate between those of covalent water molecules and discrete protons and hydroxide ions. This in turn destabilizes the structure so that it distorts on cooling in a way that cannot be explained by conventional symmetry-lowering mechanisms. The resulting synergistic hydration−distortion mechanism is, to the best of our knowledge, unique to close-packed ionic compounds.

Distinct Intercalation and Conduction Behaviors within an Isostructural Series Ba5R2Al2SnO13

“…It may even reduce the likelihood of a symmetry-lowering phase transition on cooling, by completing the close-packing of the BaO layer. This contrasts with other barium-containing hexagonal perovskites we have studied recently, namely, 6H–Ba 4 Ta 2 O 9 and Ba 4 Sb 2 O 9 , where hydration seems to stabilize the former due to symmetry-lowering. − …”

High Proton Conduction in the Octahedral Layers of Fully Hydrated Hexagonal Perovskite-Related Oxides

“…Many hexagonal perovskite-related oxides also contain cubic close-packed (c) and/or anion-deficient cubic close-packed (c′) layers. Recently, the hexagonal perovskite-related oxides have been reported to exhibit significant proton conduction without chemical doping due to the presence of intrinsic oxygen vacancies. ,,− Here, the intrinsic oxygen vacancies are the oxygen vacancies in a parent material. However, the proton conduction of hexagonal perovskite-related oxides has not been extensively studied compared to that in AB O 3−δ perovskite oxides. − Murakami et al reported Ba 5 Er 2 Al 2 ZrO 13 as a new class of proton conductors with high proton conductivity comparable to those of representative proton conductors (e.g., 10 –3 S cm –1 at 300 °C).…”