Phonon instability and mechanism of superionic conduction in Li2O

We report studies on the vibrational and elastic behavior of lithium oxide, Li2O, around its superionic transition temperature. Phonon frequencies calculated using the local density approximation and generalized gradient approximation schemes of ab initio density functional theory are in excellent agreement with the reported experimental data. Further, volume dependence of phonon dispersion relation was calculated, which indicated softening of zone boundary transverse acoustic phonon mode along [110] at the volume corresponding to the superionic transition in Li2O. This instability of phonon mode could be a precursor leading to the dynamic disorder of the lithium sublattice. Empirical potential model calculations were carried out to deduce the probable direction of vacancy-assisted lithium movement by constructing a supercell consisting of 12000 atoms. The energy profile for lithium ion movement was computed at volumes corresponding to ambient and superionic regimes. The energy considerations along various symmetry directions indicated that [001] was the most favorable direction for lithium movement in the fast ion phase. This result corroborated our observation of dynamic instability in the transverse phonon mode along the (110) wave vector. Using molecular dynamics simulations, we studied the temperature variation of elastic constants, which showed a large decrease in C-11, consistent with known experimental observation.