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 ${\mathrm{FeSi}}_2$ (${\mathrm{CaF}}_2$ 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 Å for FeSi and ${\mathrm{FeSi}}_2$, 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 ${\mathrm{FeSi}}_2$ 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 ${\mathrm{FeSi}}_2$ 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 ${\mathrm{FeSi}}_2$ 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.