Bonding and structure of Co${\mathrm{Si}}_2$ and Ni${\mathrm{Si}}_2$

Self-consistent calculations of the electronic structure of Co${\mathrm{Si}}_2$ and Ni${\mathrm{Si}}_2$ give insight into the nature of the bonding and suggest an explanation of the unique structure of these two compounds. A quasigap separates bonding and antibonding states. For Co${\mathrm{Si}}_2$ and Ni${\mathrm{Si}}_2$, $E_F$ falls in or near the quasigap. The Si atoms exhibit $s{p}^3$ hybridization, absent in many other silicides. Ionicity is negligible. Bonding is well described by a directed valence-orbital picture, except for free-electron (interstitial) character at higher energies.