Heavily doped proton-conducting perovskites: Relationships between defect structure and functional properties

Proton-conducting oxides based on perovskite structures are essential materials for intermediate-temperature electrochemical devices. However, classical systems, such as barium cerates and zirconates, are limited by the solubility of acceptor dopants. This limits the achievable concentration of protonic defects and prevents the investigation of the transport mechanisms in the concentrated defect regime. This review systematically examines the current understanding of proton transport in acceptor-doped perovskite oxides. It identifies limitations inherent to conventional materials and explores the BaSn1–xInxO3–δ system. The latter is unique, since it can accommodate high indium concentrations while retaining cubic perovskite symmetry. The exceptional solid solution formation in this system allows for access to doping levels where oxygen vacancy concentrations exceed 0.2 per formula unit. This is a condition where defect-defect interactions are inevitable, and behavior may deviate qualitatively from that described by ideal solution models in the dilute limit. The present review establishes correlations between composition, average crystal structure, local atomic arrangements, defect structure, and various physicochemical and functional properties, such as hydration thermodynamics, proton conductivity, chemical expansion, and thermomechanical behavior. Particular attention is devoted to the heavily doped compositions (x = 0.4–0.7), where pronounced local structural disorder, composition-dependent trapping phenomena, and the interplay between thermal and chemical expansion effects significantly influence macroscopic transport behaviour. The combination of experimental and computational findings establishes a framework for understanding the relationships between structure and properties in highly defective proton-conducting perovskites.