Microscopic structure of the substitutional Al defect in alpha quartz

First-principles pseudopotential calculations are reported for the lattice distortion and electronic properties of the Al substitutional defect in alp ha quartz. We determine microscopical properties of the center such as Al c oordination, symmetry of the distorted lattice and defect-induced electroni c states. The localization properties of the electronic spin density of thi s paramagnetic color center are investigated. Our results show that the spi n density is evenly distributed on the four oxygen nearest neighbors to Al in contrast to phenomenological model results.