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 of 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 Sb atom is obtained. The vibration of the lowest frequency, omega(3), (and the highest oscillator strength) is the true soft mode which lies at millimeter waves. The higher frequency, omega(2) is also temperature dependent. They all three overlap and show large absorption peak accompanied by the deep dielectric dispersion in millimeter and far IR region.