The nature of the phonon dispersion relation anomalies of IV - VI compounds

The nature of phonon dispersion relation anomalies of the IV - VI compounds in the high-symmetry phase is analysed in the framework of the vibronic theory of ferroelectricity. It is shown that the source of the phonon dispersion curve anomalies is the strong electron - phonon interaction of pseudo-Jahn - Teller type. The phonon spectra of the high-symmetry phase of PbTe are calculated for different temperatures. The calculated spectra are in agreement with experimental data. It is shown that the significant temperature dependence of the soft mode in IV - VI semiconductors is caused by the closeness of the lattice to structural instability.