Landau-type pressure dependence of the order parameter: application to the ferroelectric-paraelectric pressure-induced transition in KNbO3

Following the Landau model, the pressure–temperature dependence of the order parameter is derived. Using the Lyddane–Sachs–Teller (LST) relationship, the model is applied to ferroelectricity to deduce the pressure behaviour of the soft mode driving the transition. Comparison with experiment is made using recent data obtained on KNbO₃ under pressure over a large temperature range. The results indicate that the ferroelectric–paraelectric (FE–PE) transition observed in KNbO₃ at high pressure from ～4 to ～25?GPa is of the second-order type.