Lattice and molecular dynamics studies of superionic conductors Li₂X (X=O, S and Se)

We have performed lattice and molecular dynamics studies of Li₂X (X=O, S and Se) compounds to understand the superionic conduction behaviour. The inelastic neutron scattering measurements on Li-O have been performed across its superionic transition temperature of about 1200 K. The experimental phonon spectra shows significant changes around the superionic transition temperature, which is attributed to large ionic diffusion of lithium as well as its large vibrational amplitude of Li atoms. We show that there is a correlation between the chemical pressure (ionic radius of X atom) and the superionic transition temperature in these isostructural superionic conductors. We have determined the various possible diffusion pathways and energy barrier which leads to significant difference in the superionic transition temperature in these compounds. Further, we have also investigated the relation between Li ion conductivity with the phonon spectra of these compounds. We found that the compounds with lower phonon spectral range of Li atoms favor the superionic conduction. (author)