THEORY OF RECONSTRUCTIVE PHASE-TRANSITIONS IN CRYSTALS OF THE ELEMENTS AND RELATED STRUCTURES

A reformulation of the Landau theory of phase transitions is proposed, which applies to reconstructive phase transitions in crystals. The phenomenological approach to this category of transitions, in which the symmetries of the phases are not related by group-subgroup relationship, consists in expressing the transition order parameter in terms of density waves which depend on the variational parameter associated with the transition mechanism. The main classes of allotropic transformations in crystal of the elements displacive type and reordering type - are discussed in this framework.