Microscopic structure of the substitutional Al defect in α quartz

First-principles pseudopotential calculations are reported for the lattice distortion and electronic properties of the Al substitutional defect in $\alpha$ quartz. We determine microscopical properties of the center such as Al coordination, symmetry of the distorted lattice and defect-induced electronic states. The localization properties of the electronic spin density of this paramagnetic color center are investigated. Our results show that the spin density is evenly distributed on the four oxygen nearest neighbors to Al in contrast to phenomenological model results.