Electronic Structure and Bonding in Epitaxially Stabilized Cubic Iron Silicides

TL;DR

This study uses ab initio calculations to analyze the structural and electronic properties of epitaxially stabilized cubic iron silicides FeSi and FeSi2, revealing their bonding nature and electronic behavior.

Contribution

It provides detailed ab initio insights into the structure, electronic states, and bonding characteristics of FeSi and FeSi2, compounds recently synthesized on Si(111).

Findings

FeSi is metallic with a good match to experimental DOS.

FeSi2 shows a high density of d-states at the Fermi level, explaining its instability.

Bonding in FeSi is partially ionic, while FeSi2 exhibits covalent charge accumulation.

Abstract

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 FeSi$_2$ (CaF$_2$ structure) which have recently been grown by molecular beam epitaxy on Si(111). We obtain equilibrium bulk lattice constants of 2.72 \AA\ and 5.32 \AA\ for FeSi and 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 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 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 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.