PSEUDO-JAHN-TELLER ORIGIN OF PHONON ANOMALIES IN IV-VI COMPOUNDS

Phonon anomalies in the high-symmetry phase of IV-VI semiconductors ar e analyzed in the framework of the vibronic theory of ferroelectricity . It is shown that, in the vicinity of the ferroelectric phase transit ion, the specific features of the lattice dynamics are determined by p seudo-Jahn-Teller interactions, and the interatomic potential depends on the symmetry of the lattice distortions. The above-mentioned approa ch is used for calculating the phonon spectra of PbTe and PbS.