ELECTRONIC-PROPERTIES OF ALPHA-QUARTZ UNDER HIGH-PRESSURE AND THE TRANSITION TO THE AMORPHOUS PHASE

The electronic properties of a-quartz, at ambient pressure and in connection with the structural deformation induced by compression, are examined via first-principles calculations, based on the full-potential linearized augmented-plane-wave method. A detailed analysis of the density of states and of the charge density as a function of pressure is presented. The results at ambient pressure are in good agreement with experiments and earlier theoretical studies. We found that the forbidden energy range between Si-O bonding and O 2p nonbonding states vanishes in the region of the transition to the amorphous phase, indicating a strong hybridization between these states. This hybridization turns out to be more compatible with a different Si coordination and may lead, by local rearrangement, to the formation of a mixed phase. The fundamental band gap is found to increase with pressure.