STRUCTURAL TRANSITIONS OF MULTIPLY SOFTENING SYSTEMS

An attempt is made to relate the occurrence of several softening latti ce modes with the existence of pseudosymmetries within the prototype p hases of ferroelastics and ferroelectris undergoing a structural phase transition of second order. Systems with equitranslational transforma tions and with transformations at a non-zero wave vector are considere d. Lattice-dynamical model calculations have been performed in order t o elucidate the microscopic nature of the soft modes and the transitio n mechanism of monoclinic K2ZnBr4.