Insight into the origin of superionic conductivity from electronic structure theory

We study the electronic properties of a series of tellurides with the antif luorite structure, Ag2Te, Cu2Te and M2Te (M = Li,Na, K), to reveal the outs tanding diffusivity of silver ions. We carry out first-principles density-f unctional calculations for these systems using the linearized augmented-pla ne-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 embedd ed 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 Ag2T e and Cu2Te have remarkably different degrees of p-d hybridization. The d s tates of Ag atoms are much more weakly coupled with the p states of Te atom s and hence keep their localized nature. The activation energies for the io nic diffusion are evaluated to show that the Ag ion has a smaller barrier a s 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 c ombination of the deformability of the d shell and the weakness of the p-d hybridization. To confirm this conclusion, we also apply the LAPW calculati on to AgI in the gamma-phase.