Ab initio study of the band structures of different phases of higher manganese silicides

By means of first principles calculations, we have investigated the band structures of different phases of higher manganese silicides (MnSix with x ranging from 1.73 to 1.75). In this family, Mn11Si19, Mn15Si26, and Mn27Si47 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, Mn4Si7 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 MnSi1.75 stoichiometry provides semiconductor properties without degeneracy. The role of stacking faults in the gap reduction of higher manganese silicides is discussed.