PHASE-TRANSITIONS OF PBHPO(4)-TYPE AND PBDPO(4)-TYPE FERROELECTRICS INVESTIGATED WITH A GREENS-FUNCTION TECHNIQUE

A Green's-function technique is used to study the effects of spin-phon on interactions in PbHPO4- PbDPO4-type ferroelectrics including higher -order anharmonic terms. The renormalized energy and the damping of th e spin waves and the phonons have been evaluated. The anharmonicity ef fects play an important role in the vicinity of T-c and above T-c. The transverse dynamic structure factor exhibits three peaks due to the c oupling of the transverse soft mode and the acoustic-phonon mode. The width of the central peak is proportional to the phonon damping.