ELECTRONIC-STRUCTURE AND BONDING IN EPITAXIALLY STABILIZED CUBIC IRONSILICIDES

We present an ab initio full-potential linearized augmented-plane-wave (FLAPW study of the structural and electronic properties of the two b ulk unstable compounds FeSi (CsCl structure) and FeSi2 (CaF2 structure ) which have recently been grown by molecular-beam epitaxy on Si(111). We obtain equilibrium bulk lattice constants of 2.72 and 5.32 angstro m for FeSi and FeSi2, respectively. The density of states (DOS) of FeS i agrees well with experiment and shows metallic behavior. In agreemen t with a previous calculation, the DOS of FeSi2 shows a large density of d states at the Fermi level, explaining the instability of the bulk phase. The electron charge distributions reveal a small charge transf er from Si to Fe atomic spheres in both compounds. While in FeSi the F e-Si bond is indeed partially ionic, we show that in FeSi2 the electro n distribution corresponds to a covalent charge accumulation in the Fe -Si bond region. The reversed order of d bands in FeSi with respect to FeSi2 is understood in terms of crystal-field splitting and Fe-Fe nea rest-neighbor dd interactions in the CsCl structure, and a strong Si p -Fe d bonding in the fluorite structure, respectively.