LANDAU MODELS FOR STRUCTURAL PHASE-TRANSITIONS - ARE SOFT MODES NEEDED

The Landau model for structural, displacive transitions has been widely used. When specifically applied to second orde (only!) transitions, the lattice becomes dynamically unstable for infinitesimal displacements at the critical temperature T(c); above T(c) the frequency of the critical soft mode behaves as omega2 is-proportional-to(T-T(c)). Without justification, the so-called "soft mode" theory has then been vaguely and uncritically applied to both the design and interpretation of experiments on any displacive transformation. In fact, however, second order transitions are the exception rather than the rule. Landau models for the much more common first-order transitions are presented, and the secondary role of mode softening is clarified. In contrast to ideal second-order transitions, first-order transitions are not accompanied by dynamic instabilities, rather, they are thermodynamically unstable.