Electronic and optical properties of Ir3Si5

Semiconducting silicides have been intensively studied as potential candidates for thermoelectrical and optical applications [I]. Much attention was focused on beta-FeSi, while some other materials such as Ir,Si, remain less studied both experimentally and theoretically. Ir3Si5 crystallizes in the monoclinic structure (space group P2(1)/c) with the following lattice constants: a = 6.406 angstrom, h = 14.162 angstrom, c = 11.553 angstrom, beta = 116.69 degrees [2]. The unit cell contains 6Ir and 10Si inequivalent sites. Resistivity measurements on Ir3Si5 polycrystalline samples prepared by arc-melting have shown a gap of 1.2 eV [3]. Moreover no other semiconducting phase in the Ir-Si system was established [3]. According to optical absorption investigations carried out on Ir3Si5 polycrystalline thin films fabricated by magnetron sputtering a direct band gap of 1.56 eV has been found [4]. A high background absorption level in the gap region was also observed in that study. It should be mentioned here that the direct band gap of 1.2 eV obtained by absorption measurements has been reported in Ref. [5] while the stoichiometry of the compound was found to be IrSi1.75 and the exact crystal structure could not be determined. From the other side there is a contradiction in dispersion of bands close to the gap in Ir3Si5 as calculated by ab initio methods [6, 7]. In fact, in Ref. [6] the character of the gap is indirect displaying the gap value of 0.76 eV, whereas the direct band gap of 1.14 eV is found in Ref. [7]. In addition to that, a detailed description of the chemical bonding by analyzing density of states and crystal orbital overlap population diagrams has been also performed [6]. To our knowledge, there is no information about the dielectric function for this material. In this paper we present results of ab initio calculations of electronic and optical properties of Ir3Si5 focusing mainly on the band diagram, the imaginary and real parts of the dielectric function.