Oxygen vacancy in α-quartz: A possible bi- and metastable defect

Semiempirical cluster calculations are performed on positive (${\mathrm{E}}_1^{\prime }$ center) and neutral oxygen-vacancy defects in α-quartz. A comparison is made of the various available Hamiltonians and the use of unrestricted wave functions. The results predict that these two defects form a bistable system, exhibiting the same two metastable configurations but differing in which configuration is lower in energy. The neutral charge state favors a configuration in which the silicon atoms adjacent to the vacancy relax only slightly away from their perfect crystal positions. The positive charge state yields a minimum energy configuration with large relaxation of the silicon on the long-bond side of the vacancy. This silicon atom, in fact, moves through its basal oxygen plane and bonds with an oxygen atom, making the oxygen atom threefold coordinated. Unpaired spin localization is consistent with experimental data from electron-paramagnetic-resonance studies on irradiated quartz.