The electroreflectance and thermoreflectance spectra of SnSe crystals are investigated in polarized light with E 11 u and E 11 b. The direct energy gaps are 1.05 and 1.24 eV for light polarizations E I I b and E I I a, respectively. The polarization anisotropy of modulation spectra of SnSe is strong near the gap and becomes weak at higher energies. The structures of investigated spectra in the vicinity of the forbidden energy gap are interpreted by direct optical transitions on the high symmetry line A-I?-V in the calculated energy band structure.

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The band structures of GeSe, GeS, SnSe and SnS layered semiconductor compounds have been calculated by the EO LCAO method in a single-layer approximation. The Madelung energy of electron in the field of all lattice ions was taken into account. The results of band-structure and density-of-states calculations are in good agreement with earlier calculations carried out by the pseudopotential method. It is shown that, unlike Ge chalcogenides, the minimal energy gap for Sn chalcogenides is situated on the symmetry line Lambda (k, 0, 0) and not on V(0, 0, k). The effective masses of electrons and holes near band extrema have been calculated.

The Raman scattering spectra of CdGa 2 Se 4 under pressure were investigated at 300 K up to 20.9 GPa. TO as well as LO modes were observed. Two stages were observed in the pressure dependences of Raman bands. Such a behaviour was attributed to the order -disorder phase transition in the cation sublattice. A first-order phase transition under pressure was observed at 20.2 GPa. Using the Harrison -Keating model of the lattice dynamics modified for crystals with the tetragonal structure, the bulk modulus B and the mode-Grüneisen parameters Γ i were determined for the first time. It is shown that a better agreement between the experimental and calculated values of Γ i is observed if one takes into consideration the different behaviour with pressure for the bond-bending and the bond-stretching parameters, which determine the low-(lower than 140 cm -1 ) and high-(higher than 140 cm -1 ) frequency phonons, respectively.

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