Rare-Earth-Substituted Strontium Titanate: Insight into Local Oxygen-Rich Structures and Redox Kinetics

Abstract

Ln-substituted SrTiO3 is a promising material for energy conversion technologies such as thermoelectric generators and solid oxide fuel/electrolysis cells. In this study, formation of local structures enabling accommodation of excess oxygen in perovskite matrix of SrTiO3 and related redox behavior were assessed employing static lattice simulations in combination with experimental methods (XRD, SEM/EDS, XPS, TGA, and electrical measurements) using Sr0.90-xLn0.10TiO3±δ (Ln = Ce, Pr; x = 0–0.10) as model systems. Although strontium-vacancy formation is found to be a preferable mechanism for donor compensation in oxidized Sr(Ln)TiO3, oxygen excess still can be accommodated by extended defects quenched from high temperatures. Linear LnSr3+···Oi2– defect clusters and SrO shear planes characteristic of Ruddlesden–Popper phases are found to be the most probable extended defects enabling the accommodation of excess oxygen in oxidized titanates with Sr1–xLnxTiO3+δ cation stoichiometry. The presence of oxygen-rich local structures is shown to be strongly correlated with the faster redox kinetics and higher electrical conductivity critical for practical applications. Easy oxidation of reduced Sr1–xLnxTiO3±δ (with electronic donor compensation) provide further evidence in favor of LnSr3+···Oi2– defect clusters as mechanism of excess oxygen accommodation.