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At increasing temperature sulfate groups in LiNaSO 4  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 LiNaSO 4 . This includes the first reported single crystal X-ray investigation on the high temperature phase. The  α – β  phase transition in LiNaSO 4  is strongly first order ( T  tr  = 788 K) with a volume change of 6%. However, the  α  and  β -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  λ -like increase of specific heat suggests the onset of the SO 4 -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.

The trigonal-to-cubic phase transition in LiNaSO 4 : An X-ray and calorimetric study