QENS Investigation of Proton Diffusion in Sr/Ca-doped LaPO$_{4}$

We have investigated the diffusion dynamics of protons in hydrated~La$_{0.958}$Sr$_{0.042}$LaPO$_{4}$ and La$_{0.958}$Ca$_{0.042}$LaPO$_{4}$. These materials contain networks of tetrahedra rather than octahedra and relatively little is known about the mechanisms of proton transport in such systems. The samples were characterized by X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), and neutron powder diffraction (NPD) from room temperature to 800 $^{\circ}$C. The macroscopic and microscopic dynamics were studied using electrochemical impedance spectroscopy (EIS) and quasielastic neutron scattering (QENS). The activation energy characterizing the proton diffusion was determined in the temperature range~500-700 $^{\circ}$C using both QENS and EIS. For~La$_{0.958}$Ca$_{0.042}$LaPO$_{4}$, QENS~reveals a dynamical process that was not detected by EIS. The QENS activation energy was determined to be 0.09 eV, roughly an order of magnitude lower than the $\sim$ 1 eV inferred from EIS measurements. We will present results from the EIS, NPD and QENS analysis of the samples.