The trigonal-to-cubic phase transition in LiNaSO4:: An X-ray and calorimetric study

At increasing temperature sulfate groups in LiNaSO4 undergo rotational disorder which lead to an enhanced mobility of the cationic species. This disorder results in a phase transition above which the material is a superionic conductor. In order to understand the nature of the phase transition we have carried out extensive X-ray and calorimetric studies of LiNaSO4. This includes the first reported single crystal X-ray investigation on the high temperature phase. The alpha-beta phase transition in LiNaSO4 is strongly first order (T-tr = 788 K) with a volume change of 6%. However, the alpha- and beta-form bear a close structural relationship. Precursor effects indicate the approaching phase transition. We found a strongly anisotropic thermal expansion in the low temperature phase. The lambda-like increase of specific heat suggests the onset of the SO4-rotation well below T-tr. Short range order in the high temperature phase which considerably affects the heat capacity and the volume as well as indications for a further phase transition are discussed within the scope of Landau-theory, Finally, we introduce a transition-model combining the features of a continuous part and a discontinuous part of the phase transition.