Diffusive phase transitions in ferroelectrics and antiferroelectrics

In this paper, we present a microscopic model for heterogeneous ferroelectric and an order parameter for relaxor phase. We write a Landau theory based on this model and its application to ferroelectric PbFe${}_{1\mathrm{/}2}$Ta${}_{1\mathrm{/}2}$O${}_3$ (PFT) and antiferroelectric NaNbO${}_3$:Gd. We later discuss the coupling between soft mode and domain walls, soft mode and quasi-local vibration and resulting susceptibility function.