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

Powder x-ray diffraction has been performed on lithium sulphate, Li2 SO4, in the temperature range 300 to 1000 K and at pressures up to 7.5 GPa. The ambient pressure β phase appears stable up to 3 GPa, whereupon a slow transformation begins into a new phase, δ- Li2 SO4. This phase is characterized by broad, very low intensity Bragg peaks. Above 7 GPa and with slight heating, another phase, - Li2 SO4, is formed which shows sharp Bragg peaks. Rietveld refinement of the structure of the phase has shown it to be isostructural to the high-temperature phase III of Na2 SO4. Molecular dynamics simulations of the 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 α form of Li2 SO4.