Structural and dynamic properties of lithium sulphate in its solid electrolyte form

Abstract

The properties of solid lithium sulphate have been studied by computer simulation. At sufficiently high temperatures, the simulated crystal behaves as a solid electrolyte with lithium ion (jump) diffusion and sulphate group rotation. The atomic radial distribution functions in the rotator phase are discussed in relation to the low temperature, fully ordered, monoclinic structure and the nature of the orientational disorder of the sulphate groups is characterized in terms of tetrahedral rotor functions. The crystal structure factor is found to be sensitive to the model adopted for the charge distribution of the anions; good agreement with experimental neutron diffraction data is obtained when a charge distribution consistent with ab initio quantum mechanical calculations is used. The phase transition whereby the low temperature monoclinic structure transforms to the disordered cubic phase has been investigated by the constant pressure molecular dynamics method. The nature of the lithium ion diffusive motion and its coupling to the anion reorientation, the relaxation of the orientational order and the lattice vibrations are all briefly discussed.