Distinct Intercalation and Conduction Behaviors within an Isostructural Series Ba5R2Al2SnO13

Abstract

The series Ba₅R₂Al₂SnO₁₃ (R = In, Y, Er, Ho, Tb) has been synthesized and structurally characterized by X-ray and neutron powder diffraction. All members have oxygen-deficient 10-layer hexagonal (10H) perovskite-type structures at high temperature and gain mass on cooling equivalent to ∼0.5 oxygen atoms per formula unit, observed by both thermogravimetric analysis and the occupancy of a vacant site in the oxygen substructure refined against neutron powder diffraction data. The origin of this mass gain varies with R: for R = In, Y, Er, and Ho, it is due to water uptake via a hydroxylation mechanism to form Ba₅R₂Al₂SnO₁₃.xH₂O (x ≤ 0.5), with OH^– ions occupying the vacant site and the other proton forming a second OH^– in the oxygen substructure; while for R = Tb, it due to the oxidation of Tb³⁺ to Tb⁴⁺, with O^2– ions occupying the vacant site. These chemico-structural differences are consistent with the measured conductivity behavior of the samples, whereby Ba₅Er₂Al₂SnO₁₃ is a proton conductor in air at moderate temperatures (∼10^–4 S cm^–1 at 500 °C) while Ba₅Tb₂Al₂SnO₁₃ is a mixed oxide ionic and electronic conductor. These differences were further confirmed by X-ray absorption spectroscopy and corroborated by quasielastic neutron scattering.