ABSTRACT
Binary compounds of the IV-VI group long ago are known as the most perspective materials for the IR photo detectors and laser plants [1, 2]. They differ from the other semiconductors by the many fundamental parameters: very large dielectric permittivity and small energy band, the localization of the long wave transitions at the point L of the Brillouin Zone (BZ) with the p-symmetry of the upper valence band (UVB) and lower conduction band, and other [3, 4]. Therefore, it is not accidentally that their electronic structure and optical properties were theoretically investigated very intensively [3-9]. The interest to this problem after the long interval was greatly increased not only by the actuality of the A4B6 crystals for the applications but also by the creating of the modern theoretical methods for the electronic structure calculations [10-14]. They are popular also as the model direct-band binary compounds with the simplest crystal structure of NaCl type with the small energy band Eg ≈ 0.15-0.3 eV.
