Origin of optical anomalies in the ferroelectric phase transition region of SbSI and SbSBr crystals

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 BI, 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-1u(2). Thermal fluctuations of atoms in the xy plane induce fluctuations of anharmonicity of V(z) in the phase transition region around temperature T-c in SbSI and SbSBr. These anharmonicity fluctuations, in their turn, cause anomalies in the temperature dependence of the absorption edge in K(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.