Band structure calculations for β-FeSi2 have been performed by the linear muffin-tin orbital method within the local density approximation scheme including exchange and correlation effects. A detailed analysis of the conduction and valence band structure around high-symmetry points has shown the existence of a quasidirect band gap structure in the material. It is experimentally confirmed that between the threshold energy of optical interband transition of 0.73 eV and the first direct gap transition with appreciable oscillator strength at about 0.87 eV there is a region in which direct transition of low oscillator strength and indirect transitions overlap. That explains the tricky behavior of β-FeSi2 in experimental investigations demonstrating it to be either a direct or indirect gap semiconductor.

By means of first principles calculations, we have investigated the band structures of different phases of higher manganese silicides ͑MnSi x with x ranging from 1.73 to 1.75͒. In this family, Mn 11 Si 19 , Mn 15 Si 26 , and Mn 27 Si 47 have been found to behave like degenerate semiconductors and, at the same time, like metals because the Fermi level stays partly in the energy gap and partly in the valence band close to its top. The spin-polarized calculations have revealed that these phases can be also treated as half-metals displaying 100% spin polarization of holes at the Fermi energy. On the contrary, Mn 4 Si 7 is shown to be a semiconductor with the indirect band gap of 0.77 eV. Its dielectric function possesses some anisotropy effects with respect to different light polarizations. We have also discovered that the MnSi 1.75 stoichiometry provides semiconductor properties without degeneracy. The role of stacking faults in the gap reduction of higher manganese silicides is discussed.

Electronic band structure calculations of Nowotny ''chimney-ladder'' isostructural ruthenium and osmium silicides and related germanides have been performed by the linear muffin-tin orbital method within the local density approximation. Both silicides have been found to be direct gap semiconductors with energy gaps of 0.41 and 0.95 eV in Ru 2 Si 3 and Os 2 Si 3 , while the band gaps in the germanides have a competitive indirectdirect character with gaps of about 0.3 and 0.9 eV in Ru 2 Ge 3 and Os 2 Ge 3 , respectively.

By means of ab initio calculations we have revealed a newly discovered Ca3Si4 compound to be a semiconductor. It is characterized by an indirect transition of 0.35 eV. A peculiar dispersion of the last valence band and the first conduction band, displaying a loop of extrema, has been found. This feature leads to large anisotropy of the mobility of holes and electrons. We also present the dielectric function of this material in comparison with data for another semiconducting calcium silicide Ca2Si.

For the first time, theoretical arguments for the semiconducting properties of the ReSi1.75 phase have been given by means of ab initio linear muffin-tin orbital method (LMTO) calculations. It is shown that the material is indeed a narrow-gap semiconductor with an indirect gap value of 0.16 eV. The first direct transition with appreciable oscillator strength at 0.30 eV is predicted.