First-principles study of calcium di-silicide under high pressure

Electronic states for two possible high-pressure phases (with the ThSi2 str ucture and with the AlB2 structure) of CaSi2 are determined by means of a l ocal density approximation band-structure calculation. We confirm that thes e polymorphs can be regarded as doped sp(2) networks of Si. However, a simp le picture in terms of K-orbitals is not adequate for explaining the conduc tion bands, because (i) an s-like band, which is anti-bonding in sp2 connec tions but bonding between segments of the network, appears and (ii) there i s pi-d hybridization between Si and Ca.