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The dependence of the full electronic potential V ( z ) in paraelectric and ferroelectric phases of SbSI and SbSBr crystals upon the normal coordinate of symmetry B1, formed of atom displacements along the c ( z ) axis is studied in the vicinity of Sb atoms. The exponential dependence of the absorption coefficient K ( E ) upon the photon energy E is shown to be caused by interaction of electrons with the phonons of the normal mode B:,,. Thermal fluctuations of atoms in the xy plane induce fluctuations of anharmonicity of V ( z ) in the phase transition region around temperature T, in SbSI and SbSBr. These anharmonicity fluctuations, in their turn, cause anomalies in the temperature dependence of the absorption edge InK(E) slope. It is demonstrated using the pseudopotential approach that the variation of the direct band gap during the phase transition is mainly determined by variation of the structure form factor when Sb atoms change their equilibrium position.

SbO x S 1−x I (x = 0-0.5) crystals have been grown from the vapour phase. Reflection spectra of the SbO x S 1−x I (x = 0.2) crystals were studied by a modernized Fourier spectrometer. Using an improved Karmers-Kronig technique with two confining spectral limits, the spectra of optical parameters and optical functions were calculated. The vibrational frequencies ω L and ω T have been evaluated. The vibrational frequencies of SbSI and SbO x S 1−x I (x = 0.5) chains in different phases have been calculated in the harmonic approximation. The theoretical results are compared with experimental data. Taking into account the anharmonicity of the electronic structure and lattice, caused mainly by the phonon-phonon interaction, the phase transition temperature T C as a function of the mixture composition x has been evaluated. The dependence of the total potential energy function of the soft B 1u mode upon temperature determines variation of its frequency ω 2 s in the phase transition region.