Proton migration and defect interactions in the CaZrO3 orthorhombic perovskite:: A quantum mechanical study

Quantum mechanical techniques based on density functional theory have been used to investigate the mechanism and energetics of proton transport in the perovskite-structured CaZrO3. The calculations demonstrate that the observed orthorhombic crystal structure (comprised of tilting [ZrO6] octahedra) is reproduced accurately. Quantum mechanical molecular dynamics simulations confirm that the diffusion mechanism involves proton transfer from one oxygen ion to the next (Grotthuss-type mechanism) and also indicate the importance of the vibrational dynamics of the oxygen sublattice. For each hopping event, the oxygen-oxygen distance contracts to about 2.4-2.5 Hi so as to assist proton transfer. By exploration of the energy profiles for proton transfer, a very low energy barrier is found for the O(1)-O(1) interoctahedra path. However, long-range proton conduction may involve O(1)-O(2) proton transfer as the rate-limiting step with a calculated energy barrier of 0.74 eV. Binding energies for hydroxyl-dopant pairs involving Ga3+, Sc3+, and In3+ dopant ions are predicted to be favorable and are compatible with observed proton "trapping" energies from previous muon spin relaxation and quasi-elastic neutron scattering experiments.