Ab-initio electronic structure studies of mobility paths in fast-ion conductors-I. Results for MI₄^-3 clusters

We report results of minimum-basis Pseudopotential Hartree-Fock studies of MI₄^-3 clusters (M = Na⁺, K⁺, Ag⁺, and Cu⁺), and of HgI₄^-2. The calculations are designed to characterize local-site effects on mobility paths in solid state electrolytes. We observe qualitatively correct behavior, with Ag⁺ predicted to be the most mobile ion. Quadrupolar polarizability of the metal ion, which is produced by s-d mixing, lowers the energy of trigonal transition state, thus accounting for the observation that quadrupole polarizable species are ideal mobile ions in close-packed halide frameworks. Mulliken populations show that there is considerable local covalency, so that electrostatic potential studies must be done very carefully. Expansion of the I₄ tetrahedron lowers the barrier energy.