Insight into the origin of superionic conductivity from electronic structure theory

Abstract

We study the electronic properties of a series of tellurides with the antifluorite structure, and (M = Li, Na, K), to reveal the outstanding diffusivity of silver ions. We carry out first-principles density-functional calculations for these systems using the linearized augmented-plane-wave (LAPW) method, where the exchange-correlation effects of electrons are treated in the local density approximation. The calculations illuminate the difference in the outermost electron configurations between the noble-metal (d-shell) and the alkali (sp-shell) ions. The noble-metal ions embedded in the Te sublattice with the p valence band are far more deformable in the crystalline field than the alkali ions are. We then elucidate that and have remarkably different degrees of p-d hybridization. The d states of Ag atoms are much more weakly coupled with the p states of Te atoms and hence keep their localized nature. The activation energies for the ionic diffusion are evaluated to show that the Ag ion has a smaller barrier as compared with the alkali and copper ions in the Te framework. It is thus inferred that the superionic conductivity of Ag ions primarily stems from combination of the deformability of the d shell and the weakness of the p-d hybridization. To confirm this conclusion, we also apply the LAPW calculation to AgI in the -phase.