Surface effects on atomic diffusion in a superionic conductor: A molecular dynamics study of lithium oxide

The changes induced by a surface in the atomic diffusion in Li2O are studied by molecular dynamics, using a rigid-ion potential model fitted to ab initio data. The properties of the {111} surface are investigated plane by plane at a temperature selected in the superionic phase. The Frenkel defect atomic-fraction and the lithium-ion migration are enhanced mainly in the topmost two planes. The equality of the interstitial and vacancy atomic-fractions found in the bulk is not observed in these planes. The surface migration predominantly occurs by two types of nearest-neighbor atomic jumps via a vacancy mechanism. A geometrical model of the atomic jump-frequency profile allows us to estimate the reduction of the vacancy migration-energy at the surface. We also find a spontaneous 1 × 2 reconstruction of the {110} surface.