High-pressure forms of lithium sulphate: Structural determination and computer simulation

Abstract

Powder x-ray diffraction has been performed on lithium sulphate, ${\mathrm{Li}}_{2}{\mathrm{SO}}_{4}$, in the temperature range 300 to 1000 K and at pressures up to 7.5 GPa. The ambient pressure $\ensuremath{\beta}$ phase appears stable up to 3 GPa, whereupon a slow transformation begins into a new phase, $\ensuremath{\delta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{SO}}_{4}$. This phase is characterized by broad, very low intensity Bragg peaks. Above 7 GPa and with slight heating, another phase, $ϵ\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{SO}}_{4}$, is formed which shows sharp Bragg peaks. Rietveld refinement of the structure of the $ϵ$ phase has shown it to be isostructural to the high-temperature phase III of ${\mathrm{Na}}_{2}{\mathrm{SO}}_{4}$. Molecular dynamics simulations of the $ϵ$ phase using an established potential indicate disordering of the lithium ions and rotations of the sulphate groups at high temperature, but not at a level approaching the extreme disordering shown in the superionic $\ensuremath{\alpha}$ form of ${\mathrm{Li}}_{2}{\mathrm{SO}}_{4}$.