Electronic structure and properties of pure and doped ε-FeSi from ab initio local-density theory

Local-density calculations of the electronic structure of FeSi, FeSi_1-xAl_x, and Fe_1-xIr_xSi systems in the B20 structure are presented. Pure FeSi has a semiconducting gap of 6 mRy at 0 K. Effects of temperature (T) in terms of electronic and vibrational excitations are included. Various measurable properties, such as magnetic susceptibility χ(T), electronic specific heat C(T), thermoelectric power S(T), relative variations in resistivity ρ(T), and peak positions in the density of states (DOS) are calculated. The feedback from vibrational disorder onto the electronic structure is found to be essential for a good description of most properties, although the results for S(T) in undoped FeSi can be described up to about 150 K without considerations of disorder. Above this T, only the filling of the gap due to disorder accompanied by exchange enhancement, can explain the large susceptiblity. The overall good agreement with experimental data for most properties in doped and pure FeSi suggests that this system is well described by local-density approximation even at large T. Doped FeSi can be described quite well from rigid-band shifts of the Fermi energy on the DOS of pure FeSi. Spin polarization in Ir-doped FeSi leads to a semimetallic magnetic state at low T.