Order parameter behaviour and thermal hysteresis at the phase transition in the superionic conductor lithium sodium sulfate LiNaSO4

Investigation of the thermal expansion behaviour of the superionic conductor lithium sodium sulfate, LiNaSO4 reveals coupling of the spontaneous strain with the order parameter in the temperature-induced phase transition. Landau theory provides a proper tool for the quantitative analysis of the strongly first order bcc↔P31c transition. Apart from the large order parameter step Qo=0.7 at Ttr, the origin of the first order discontinuity is not due to strain–coupling. Similar to diffusionless martensitic transformations, LiNaSO4 exhibits an athermal behaviour, i.e. the transition proceeds instantaneously as a function of temperature rather than isothermally. The thermal hysteresis may be understood in terms of self-pinning of the coherent interface between the low and the high temperature phase.