Distinct Intercalation and Conduction Behaviors within an Isostructural Series Ba₅R₂Al₂SnO₁₃

Abstract

The series Ba 5 R 2 Al 2 SnO 13 (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 Ba 5 R 2 Al 2 SnO 13 .xH 2 O (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 Tb 3+ to Tb 4+ , with O 2− ions occupying the vacant site. These chemico-structural differences are consistent with the measured conductivity behavior of the samples, whereby Ba 5 Er 2 Al 2 SnO 13 is a proton conductor in air at moderate temperatures (∼10 −4 S cm −1 at 500 °C) while Ba 5 Tb 2 Al 2 SnO 13 is a mixed oxide ionic and electronic conductor. These differences were further confirmed by X-ray absorption spectroscopy and corroborated by quasielastic neutron scattering.