Splitting of the soft mode in a double well potential of SbSI

Our latest results for ferroelectric phase transition in SbSI are presented . The electronic potential of the soft B-1u, mode above T-c at Sb site is o f the form of a double well. Under the influence of the anharmonicity, the threefold splitting of the frequency of the local normal vibration of the S b atom is obtained. The vibration of the lowest frequency, omega(3), (and t he highest oscillator strength) is the true soft mode which lies at millime ter waves. The higher frequency, omega(2) is also temperature dependent. Th ey all three overlap and show large absorption peak accompanied by the deep dielectric dispersion in millimeter and far IR region.