Superionic behavior of lithium oxide Li2O:: A lattice dynamics and molecular dynamics studyx -: art. no. 174307

We report a cumulative study of lithium oxide in its normal as well as superionic phase using both lattice dynamical and molecular dynamical calculations. Molecular dynamics simulations have been carried out to study the fast ion phase and the diffusion behavior of lithium and oxygen ions. The results obtained for the diffusion constant and the thermal amplitude of lithium are in very good agreement with experimental observations. The pair correlation functions, Li-O-Li bond angle distribution and snapshots of the positions of lithium atoms over a range of time steps, provide a microscopic picture of the local structure indicating that as in other fluorites, the lithium ions diffuse via an interstitial mechanism but the distortions due to this movement are small. Lattice dynamical calculations have been done using a shell model in the quasiharmonic approximation. We have calculated the equilibrium structure, phonon frequencies, the elastic constants and the specific heat, which are in excellent agreement with the available experimental data. We also predict the pressure variation of the phonon dispersion and the equation of state.