FE, RU, AND OS DISILICIDES - ELECTRONIC-STRUCTURE OF ORDERED COMPOUNDS

Electronic structure properties of MSi(2), where M=Fe, Ru, or Os, in t he orthorhombic beta phase are investigated using the linear muffin-ti n orbital method in the atomic sphere approximation. Selective substit ution of Fe with Ru, Os, and Cr in beta-FeSi2 is also studied. These c ompounds are small-gap semiconductors with theoretical energy band gap s ranging from 0.06 to 0.50 eV, with the exception of the metallic Cr- substituted disilicides. Substitution of Ru or Os for Fe in beta P-FeS i2 leads to a reduction of the gap width, an increase in volume of the unit cell, and a bulk modulus similar to or slightly smaller than for beta-FeSi2. Although the theoretical lattice constant of beta-OsSi2 a grees well with experiment, the calculated band gap (0.06 eV) is much smaller than the band gap in beta-FeSi2. This strongly contrasts with experimental observations that suggest a larger band gap in beta-OsSi2 (1.4 - 1.8 eV). Consideration of a proposed metastable monoclinic for m of OsSi2 does not remedy this discrepancy, since it is found to be a semimetal.