ABINITIO SELF-CONSISTENT-FIELD MOLECULAR-ORBITAL CALCULATIONS ON DEFECTS ASSOCIATED WITH RADIATION-DAMAGE IN ALPHA-QUARTZ

A series of ab initio self-consistent-field molecular orbital (SCF-MO) calculations, on defect centers involved in radiation damage of alpha quartz, is reported. Calculations were performed on a 21 atom cluster (Si5O16(12-)), using a 3-21 G basis set. The long-range Coulomb interactions of alpha quartz were included, by surrounding the 21 atom cluster with 956 point charges, so that on geometry optimization, the characteristic features of alpha quartz were maintained. Various defect centers containing Al3+ substitutional ions, and their compensating electron-hole and proton defects were studied. The results obtained for the [AlO4]0 center are in close agreement with electron spin resonance (ESR) data. Calculations were also performed on the oxygen vacancy and related defects, such as the E1' and E4' defects, where qualitative agreement with ESR results was obtained. An assessment is made, of the effect of including long-range Coulomb interactions, by comparison with other SCF-MO cluster models of these systems, which do not include these electrostatic effects.