ALL-ELECTRON STUDY OF THE ELECTRONIC-PROPERTIES OF QUARTZ WITH AL SUBSTITUTIONAL IMPURITY

An aluminum defect in quartz has been studied by means of all-electron calculations based on the full-potential linearized-augmented-plane-w ave method. A comparative study of alumina, pure quartz, and Al-substi tuted quartz shows that the Al impurity introduces levels between the bonding and nonbonding states of pure SiO2 alpha quartz; these feature s are shown to derive from the formation of the Al-O chemical bond. Th e dangling bond in the [Al(O-4)(1/2)](-) unit is evenly shared among t he four oxygen atoms adjacent to the defect, and no bias of the charge density around the impurity is observed. A planar averaged potential energy curve for a light carrier ion (e.g., Li+, H+), trapped in an op tical channel of the defective quartz has been determined as a functio n of its location with respect to the aluminum position.