SOFT MODES AT FERROELASTIC PHASE-TRANSITIONS

Ferroelastic transitions have been classified by Aizu according to the genuine source of their instability. Ferroelastics of the acoustic ty pe display an intrinsic soft acoustic mode whereas ferroelastics of th e optic type become elastically soft by a bilinear coupling between so ftening optic modes and strain. Some crystals undergoing transitions o f the latter kind at the center of the Brillouin zone are surveyed. Va rious behaviours of Raman active soft modes are encountered in systems of different complexity. Implications for the elastic properties and for domain structures are demonstrated.