ORDER-PARAMETER SATURATION AND LOW-TEMPERATURE EXTENSION OF LANDAU THEORY

The temperature evolution of the structural order parameter in displacive phase transitions with high transition temperatures (e.g. quartz, As2O5, LaAlO3, CaCO3, NaNO3, Pb3 (PO4)2 etc.) follows a Landau-type behaviour over large temperature intervals. Below a saturation temperature T(s), however, the order parameter tends to become temperature independent. A quantitative analysis shows the correlation k(B) T(s) = h-OMEGA/4 where OMEGA is the frequency of the relevant local excitation at T(s). A general Landau-type expression for the Gibbs energy is given which includes the effects of order-parameter saturation.