LATTICE MELTING AT STRUCTURAL PHASE-TRANSITIONS

Lattice melting is an unusual effect that occurs at the continuous str uctural phase transition belonging to the m = 2 universality class. Re normalization group theoretical studies have shown that the mean-squar ed atomic displacements are expected to diverge at this transition, mu ch as they do in a conventional liquid-solid transition. However, in t he m = 2 transition they diverge continuously, and only along one crys tallographic direction. The end result is still that the crystalline l ong range order is destroyed completely at T-c, but unlike a conventio nal melting transition it recovers after the transition is passed. The current theoretical understanding of this effect is reviewed together with a discussion of the search for an experimental realization. Rece nt experiments that show that the ferroelastic transition in anhydrous Na2CO3 is the first known to show ideal lattice melting are detailed.