Electronic structure of the paramagnetic boron oxygen hole center in B-dopedSiO2

Abstract

We have studied the ground-state properties of boron-related dia- and paramagnetic point defects in B-doped silica. Hartree-Fock and density functional theory calculations have been performed to determine the structure, charge, and spin distribution of the boron oxygen hole center (BOHC). The currently accepted model of the BOHC is that of a hole localized on a nonbonding $2p$ orbital of an O atom in a bridge position between a B and a Si atom, $\ensuremath{\equiv}{\mathrm{B}\mathrm{---}\mathrm{O}}^{\mathbf{\ensuremath{\cdot}}}---\mathrm{Si}\ensuremath{\equiv}.$ Our calculations do not support this model and show that the structure is not stable and spontaneously evolves into a planar trigonal diamagnetic boron center, $g\mathrm{B}---,$ and a nonbridging oxygen, $\ensuremath{\equiv}{\mathrm{Si}\mathrm{---}\mathrm{O}}^{\mathbf{\ensuremath{\cdot}}}.$ The results of this study suggest that the BOHC consists of a three-coordinated B atom bound to a non bridging oxygen, $g\mathrm{B}---{\mathrm{O}}^{\mathbf{\ensuremath{\cdot}}}.$ The computed hyperfine coupling constants for this model are in quantitative agreement with those measured experimentally for B-doped silica. This assignment is consistent with recent magnetic resonance studies on borosilicates and alkali borate glasses.