High-pressure Forms of Lithium Sulphate: Structural Determination and Computer Simulation

Powder x-ray diffraction has been performed on lithium sulphate, Li(2)SO(4), in the temperature range 300 to 1000 K and at pressures up to 7.5 GPa. The ambient pressure beta phase appears stable up to 3 GPa, whereupon a slow transformation begins into a new phase, delta-Li(2)SO(4). This phase is characterized by broad, very low intensity Bragg peaks. Above 7 GPa and with slight heating, another phase, epsilon-Li(2)SO(4), is formed which shows sharp Bragg peaks. Rietveld refinement of the structure of the epsilon phase has shown it to be isostructural to the high-temperature phase III of Na(2)SO(4). Molecular dynamics simulations of the epsilon 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 alpha form of Li(2)SO(4).