Ab initio self-consistent-field molecular-orbital calculations on AlO4 centres in alpha-quartz. II.

Abstract

In this paper, the second of a series devoted to developing quantitative understanding of defect species in silicates, we report ab initio self-consistent-field molecular-orbital calculations done on 10-atom and 22-atom clusters designed to simulate cation-compensated aluminum impurity centres [AlO4/M+]qt in alpha-quartz; here qt = 0 or + 1, and M = H, Li, or Na. The geometric configurations of the small models of these centres were optimized. The calculated values for total energies, various static structural parameters, electron-spin densities, net atomic charges, and orbital energies are compared with experimental data, especially those derived from electron-paramagnetic-resonance studies of hole centres and give good agreement. For example, the models predict that the holes will occur at opposite types of oxygen ions adjacent to Al3+, respectively, for uncompensated centres [AlO4]0 and for the centres [AlO4/M+]+, as is, in fact, observed. Some calculations for interstitial cations M+ and atomic hydrogen on clusters simulating large-channel c-axis oxygen sites of alpha-quartz are reported. Various reaction energies involving the aluminum centres and the interstitial species are listed.