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 rece ntly been solved and assigned both to the Cmca space group with nearly iden tical axial ratios and atomic coordinates. In order to investigate the bond ing properties of these two phases we have performed self-consistent densit y-functional linear muffin-tin orbital calculations of the electronic struc ture. We find for Si-VI that the band structure and density-of-states (DOS) are nearly perfectly free-electron-like, although features reminiscent of cr-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 r esults in a strongly structured DOS. The charge distribution in Cs-V shows features which can be interpreted in terms of multi-center d-electron bondi ng. (C) 2000 Elsevier Science Ltd. All rights reserved.