  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$_{1/2}$Ta$_{1/2}$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.
