Order parameter saturation at low temperatures -numerical results for displacive and O/D systems

The temperature dependence of the order parameters in quartz, NaNO3, Mo8O23 show saturation behaviour with characteristic saturation temperatures θ. The quantitative behaviour is in good agreement with the predictions of Salje, Wruck and Thomas (1991) for both displacive (eg. quartz) and order/disorder systems (eg. NaNO3). The saturation temperatures are approximately half the thermodynamic Einstein temperatures θs ≈ ½ θE indicating the relevance of the heat bath for the fluctuations. Arguments are given for the applicability of the 'displacive' Salje, Wruck, Thomas approach also for order/disorder systems.