Electronic structure and bonding in epitaxially stabilized cubic iron silicides.

We present an ab initio full-potential linearized augmented plane-wave (FLAPW) study of the structural and electronic properties of the two bulk 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 Å and 5.32 Å for FeSi and FeSi2, respectively. The density of states (DOS) of FeSi agrees well with experiment, and shows metallic behavior. In agreement 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 transfer from Si to Fe atomic spheres in both compounds. While in FeSi the Fe-Si bond is indeed partially ionic, we show that in FeSi2 the electron 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 nearest neighbor dd-interactions in the CsCl structure, and a strong Si p/Fe d bonding in the fluorite structure, respectively. Typeset using REVTEX