INCOMMENSURATE PHASE-TRANSITIONS AND OPTICAL-ACTIVITY

Recent experiments with the high-accuracy universal polarimeter method have revealed optical activity of the incommensurate phases of A2BX4 crystals. The origin of this important phenomenon is explained. A pure imaginary perturbation that takes place in the Hermitian dynamical matrix induces two circularly helical lattice waves. One of them is condensed in the same form in the incommensurate phase. This is also the physical picture of the existence of the phason mode. The symmetry of the crystal becomes D which permits optical activity. The theory is consistent with the Lifshitz condition and with our experimental results. © 1990 The American Physical Society.