Electronic structure and bonding in the Cmca phases of Si and Cs

The crystal structures of the high-pressure phases Cs-V and Si-VI have recently been solved and assigned both to the Cmca space group with nearly identical axial ratios and atomic coordinates. In order to investigate the bonding properties of these two phases we have performed self-consistent density-functional linear muffin-tin orbital calculations of the electronic structure. We find for Si-VI that the band structure and density-of-states (DOS) are nearly perfectly free-electron-like, although features reminiscent of σ-type bonds made up of sp orbitals are evident from the valence electron charge density. In Cs-V the pressure-driven electronic s→d transition results in a strongly structured DOS. The charge distribution in Cs-V shows features which can be interpreted in terms of multi-center d-electron bonding.