Asymmetry and long-range character of lattice deformation by neutral oxygen vacancy in α-quartz

Abstract

The neutral oxygen vacancy in ${\mathrm{SiO}}_{2}$ is important both through its role in controlled refractive index changes and as an archetypal intrinsic defect. We have studied the very significant effects of lattice relaxation on the structure and properties of this defect in both pure and Ge-doped \ensuremath{\alpha}-quartz using a hybrid classical--ab initio embedded-cluster method. The neutral vacancy induces very strong and anisotropic lattice distortion. At the vacancy site, the Si-Si distance in \ensuremath{\alpha}-quartz relaxes to the same spacing as in elemental Si. The long-range distortion components extend further than 13 \AA{} from the vacant site. The displacements of surrounding atoms are strongly asymmetric with respect to the vacancy, contrary to previous theoretical results. We predict a strong relaxation in the lowest triplet excited state of the vacancy and small (less than 1 eV) triplet luminescence energy. The strong dependence of the defect properties on the radius of the relaxed region is demonstrated and the applicability of small molecular cluster models is discussed.