Proton-transfer mechanism in LaPO4

On the basis of the density functional calculations, the proton-transfer mechanism in LaPO4 has been investigated at the atomic scale within the transition state theory. Through the use of the nudged elastic band method, the minimum energy paths of typical proton-transfer processes were computed, including intratetrahedral transfer, intertetrahedral transfer, proton rotation around PO4 tetrahedra, and the oscillatory transfer between adjacent tetrahedra. Because the PO4 tetrahedra in LaPO4 are separated, proton transfer between different tetrahedra is required for continuous proton conduction. The energy barriers for these processes are shown to be around 0.8 eV, very close to experimentally measured activation energies. The intratetrahedral transfer on the phosphate groups presents a high-energy barrier and is very unlikely to occur. Proton conductivity of LaPO4 was also calculated by considering the vibration frequencies of the system within the harmonic approximation.