In Situ Neutron Diffraction of Rare-Earth Phosphate Proton Conductors Sr/Ca-doped LaPO$_4$ at Elevated Temperatures*

Acceptor-doped lanthanum orthophosphates are potential candidate electrolytes for proton ceramic fuel cells. We combined neutron powder diffraction (NPD) at elevated temperatures up to $800\,^\circ\rm{C}$, X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) to investigate the crystal structure, defect structure, thermal stability and surface topography. NPD shows an average bond length distortion in the hydrated samples. We employed Quasi-Elastic Neutron Scattering (QENS) and electrochemical impedance spectroscopy (EIS) to study the proton dynamics of the rare-earth phosphate proton conductors 4.2\% Sr/Ca-doped LaPO$_4$. We determined the bulk diffusion and the self-diffusion coefficients. Our results show that QENS and EIS are probing fundamentally different proton diffusion processes. \newline 1. A. B. al-Wahish et al., Chem. Mater. 28, 7232 (2016). \newline 2. A. B. al-Wahish et al., Rev. Sci. Instrum. 86, 095102 (2015). \newline 3. A. B. al-Wahish et al., J. Phys. Chem. C. 118, 20112 (2014).