LANDAU THEORY REVISITED

The Landau free energy G(L)(T,Q) is an important concept in structural phase transitions because it is often the meeting point of experiment and the development of microscopic models. We review recent work on the real Q and T dependence of G(L), the origin of non-classical noncritical exponents for T below T(C), the cross-over from soft mode to O/D behaviour, local bifurcation at T* and relation to ordering kinetics. Framework structures such as many silicates and the perovskite structure allow special geometrical Rigid Unit phonon Modes. The material is then likely to be near to the soft mode limit, resulting in classical behaviour of Q(T) over a very wide T range. Long range correlations result in a very small Ginzburg interval.