EPR study of substitutional-aluminum-related hole centers in synthetic α-quartz

Abstract

The analysis of the anisotropic EPR spectra at 77°K of three unstable paramagnetic centers with 27Al hyperfine structure in x-irradiated a-quartz is presented. These centers are described by models in which a hole is localized in a nonbonding orbital of an oxygen which is adjacent to an aluminum impurity in a silicon lattice site; there is an interstitial ion H +, Na +, or Li + associated with this configuration. The structure of these centers is related to the stable compensator-free substitutional-aluminum center reported by Griffiths, Owen, and Ward. Two additional unstable centers with 27Al hyperfine structure, which are observed at lower concentrations after x-irradiation at 77°K, are reported; one of these centers is a second species with H +-compensation while the other is apparently another compensator-free center. The EPR spectra of quartz plates grown on a major rhombohedral seed show a strong nonrandomness in the distribution of aluminum among the three silicon sites of the unit cell. The preferred site was determined by relating the angular dependence of the spectra to the atomic arrangement in the sample. Using the identification of this site, the directions of the principal EPR interactions were related directly to the bond directions at the paramagnetic center.