Origin and nature of the band gap in β-FeSi2

The recent interest in β-${\mathrm{FeSi}}_2$, the semiconductor phase of metallic iron silicides, is related to the nature of its gap. By semiempirical tight-binding calculations we investigate the formation of the gap and the changes in the cohesion energy when the original calcium-fluorite structure (bulk unstable and metallic) is distorted into the β phase. We find out that the latter corresponds to an indirect semiconductor, but minor deformations of the structure turn the gap into a direct one.