First-principles calculations of proton defect properties in Ca-doped YPO4

YPO₄ has a xenotime-type structure with one-dimensional percolating channels along the c-axis, and Ca-doped YPO₄ exhibits proton conduction. In the present study, using first-principles calculations, we investigate the behaviors of proton solutions in 3 mol% Ca-doped YPO₄ in the intermediate temperature range on the basis of point-defect formation energies, defect concentrations, and migration barriers. Although the charge-neutrality condition is mainly satisfied by Ca^′_Y and OH^•_O defects within the defect formation energy diagrams, the Ca^′_Y + OH^•_O defect complex has the lowest formation energy and the highest concentration among examined defects under various temperature and partial-pressure conditions. The migration barriers of isolated protons in the [100] and [001] directions, as obtained from nudged elastic band (NEB) calculations, are 0.49 and 0.17 eV, respectively, confirming that protons in the YPO₄ crystal exhibit anisotropic diffusion and are likely to migrate along the c-axis channels. The activation energy estimated by the sum of the migration energy and the association energy for the Ca^′_Y + OH^•_O defect complex is comparable to the reported experimental value. Based on concentration calculations and diffusion-property analyses, we identified the Ca doping conditions and temperature ranges that govern proton conduction in YPO₄ and elucidated the diffusion pathways.