Electronic structure, spectra, and properties of 4:2-coordinated materials. I. Crystalline and amorphousSiO2andGeO2

A two-parameter tight-binding theory of the electronic structure of 4:2-coordinated materials is proposed. The parameters, a covalent and a polar energy, are fitted to the optical absorption spectra. The valence energy bands and density of states are calculated. In terms of these a consistent interpretation of all the observed photoemission and x-ray-emission spectra of ${\mathrm{SiO}}_2$ is obtained. The x-ray-absorption spectra are also analyzed. A bond-orbital approximation allows a simple calculation of the refractive index (or dielectric constant) of the various allotropic forms of silica and germania. Finally, the variation in total energy and charge distribution with local distortion is analyzed in order to study structural stability, elastic rigidity, and the effective charges (including dynamic contributions) which determine the piezoelectric constants and infrared absorption intensities.