In Situ X-ray and Neutron Diffraction of the Rare-Earth Phosphate Proton Conductors Sr/Ca-Doped LaPO₄ at Elevated Temperatures

We investigated the crystal structure, defect structure, and thermal stability of the rare-earth phosphate proton conductors (La,M)PO₄ (where M = Sr, Ca) and obtained the thermal expansion coefficients, surface topography, size distribution, and proton conductivity. The study employed neutron powder diffraction (NPD) at elevated temperatures up to 800 °C, combined with powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). Although the proton-oxygen site is located on the corners of the PO₄ tetrahedra, the NPD shows an average bond length distortion in the hydrated 4.2% Sr/Ca-doped LaPO₄. We investigated the proton dynamics by EIS and previously by Quasi-Elastic Neutron Scattering (QENS), and determined the bulk diffusion and the self-diffusion coefficients. Our results showed that QENS and EIS probe fundamentally different proton diffusion processes, where the EIS data reflect long-range intertetrahedral diffusion, whereas QENS probes more local diffusive motions.