Structural, electronic and optical properties of Ru2Si3, Ru2Ge3, Os2Si3 and Os2Ge3

We have performed a comparative study of structural, electronic and optical properties of Ru2Si3, Ru2Ge3, Os2Si3 and Os2Ge3 by means of ultrasoft pseudopotential and full-potential linearized augmented plane wave methods. The estimated difference in the cohesion energy between the low-temperature orthorhombic phase and the high temperature tetragonal one for all these compounds indicates that the former phase is lower in energy with respect to the latter one. All materials in the orthorhombic structure are found to be direct band-gap semiconductors, still some of them in the tetragonal structure display an indirect nature (Os2Si3) or a competitive direct-indirect character (Ru2Ge3) of the gap. Optical properties are discussed by analyzing the imaginary part of the dielectric function and the dipole matrix elements corresponding to different interband transitions indicating for osmium silicide and germanide the presence of low-energy transitions with an appreciable value of the oscillator strength.