Liquidlike Cu atom diffusion in weakly ionic compounds Cu2S and Cu2Se

Copper sulphide (${\mathrm{Cu}}_{2}\mathrm{S}$) and copper selenide (${\mathrm{Cu}}_{2}\mathrm{Se}$) are known to exist in solid (S/Se)-liquid (Cu) hybrid phases which exhibit favorable thermoelectrics properties. The diffusion characteristics and its mechanism in these systems are therefore of significant interest. In this paper, we analyze these properties through examining the atomic radial distributions, mean-square displacements, and velocity autocorrelations obtained from ab initio molecular dynamics simulations. Exceptionally high Cu diffusion coefficients with values over ${10}^{\ensuremath{-}5}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}$/s are obtained, indicating the unexpected liquidlike behavior of Cu in these weakly ionic compounds. The diffusion mechanism obtained through analysis of the Cu atomic trajectories is found to be at variance with the previously proposed Chudley-Elliott jump diffusion model. In addition, tunability of these diffusion coefficients via small changes in the stoichiometry, namely, Cu deficiencies, is demonstrated. The higher number of low-frequency acoustic phonon modes associated with Se sublattices, compared to those with S sublattices, correlates well with the experimentally observed thermal conductivity difference between ${\mathrm{Cu}}_{2}\mathrm{S}$ and ${\mathrm{Cu}}_{2}\mathrm{Se}$.