Ab initio SCF-MO calculations on AlO4 centres in alpha-quartz. I.

Abstract

Ab initio SCF-MO calculations were done on nine-atom and fifteen-atom clusters designed to simulate alpha-quartz, i.e., a [SiO4]0 unit, and aluminum impurity centres [AlO4]−, [AlO4]0, and [AlO4]+ in quartz, terminated with protons or SiH3 groups. The geometry of the small model of each of these centres was optimized. Using these models and the enlarged versions thereof, calculated values for relative total energies, various static structural parameters, electron spin densities, net atomic charges, and orbital energies are compared with experimental data from spectroscopic and other techniques, especially high resolution electron paramagnetic resonance. The calculated results agree remarkably well with experiment. Thus it is evident that small clusters of atoms can be used successfully to simulate the local surroundings of a substitutional Al ion in quartz, i.e., the short-range effects are the most important.