Pressure dependence of the optical properties and the band structure of the copper and silver halides

Abstract

The absorption edge of CuCl, CuBr, CuI, and AgI is investigated with a diamond anvil cell as a function of pressure up to 16 GPa. The measurements, which reflect the various phase transitions undergone by these materials, yield the pressure coefficients of the lowest gaps. Particular attention is paid to the high-pressure rocksalt modifications, which are shown to have an indirect absorption edge in contrast to the direct edge of the sphalerite phases. This fact results from the influence of the different lattice symmetries on the halogen-$p$---metal-$d$ hybridization in the valence bands. In order to interpret the data quantitatively, we have performed self-consistent local-density calculations with the linear combination of muffin-tin-orbitals---atomic-sphere approximation method and non-self-consistent Korringa-Kohn-Rostoker calculations with the standard muffin-tin potential. The pressure coefficients so obtained agree reasonably with the experimental ones. The gaps obtained by the self-consistent potential (with no adjustable parameter) are about 2 eV smaller than the experimental ones, thus reflecting shortcomings of the local-density method for excitation properties.