Electronic structure of the high-pressure modifications of CuCl, CuBr, and CuI

The absorption spectra of edge excitons in thin films of CuCl, CuBr, and CuI have been measured as a function of hydrostatic pressure using the diamond-anvil technique. The measurements yield the pressure dependence of several exciton energies in the zinc-blende, the rhombohedral, and the tetragonal modifications of the copper halides. The experimental data are compared with band-structure calculations performed with the linear-muffin-tin-orbital method. The valence bands of the rhombohedral and tetragonal phases of the copper halides are strongly influenced by the hybridization of copper 3d and halogen p states. This p-d hybridization is allowed by symmetry for all points of the Brillouin zone in all modifications studied. The edge excitons are interpreted as related to direct transitions at k=0 between valence bands split by ligand-field and spin-orbit effects and an orbital singlet conduction band.