ANHARMONIC EFFECTS IN LHP-TYPE AND LDP-TYPE FERROELECTRICS, INVESTIGATED WITH THE GREENS-FUNCTION TECHNIQUE

A Green's function technique is used to study the anharmonic pseudo-sp in-phonon and phonon-phonon interaction effects in LHP and LDP-type fe rroelectrics on optical phonon mode frequency, damping and transverse dynamical structure factor below and above T-c. The coupling between t he transverse soft-mode and the optical phonon mode produces a central peak in the transverse dynamical structure factor. The temperature de pendence of these quantities is discussed and it is in good agreement with the experimental data. At low temperatures the contribution of th e anharmonicity is very small, but at temperatures near the critical t emperature T-c and above T-c it increases strongly.