A study was made of the ground-state properties of B-related diamagnetic and paramagnetic point defects in B-doped silica. Hartree-Fock and density functional theory calculations were performed to determine the structure, charge, and spin distribution of the B-O hole center. The currently accepted model of the B-O hole center was 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, ≡B-O-Si≡. The calculations do not support this model and showed that the structure was not stable and spontaneously evolves into a planar trigonal diamagnetic B center, >B-, and a non-bridging O, ≡Si-O·. The results suggested that the B-O hole center consisted of a 3-coordinated B atom bound to a non-bridging O, >B-O·. The computed hyperfine coupling constants for this model were in quantitative agreement with those measured experimentally for B-doped silica. This assignment was consistent with recent magnetic resonance studies on borosilicates and alkali borate glasses.

Electronic Structure of the Paramagnetic Boron Oxygen Hole Center in B-Doped SiO2. G.Pacchioni, M.Vezzoli, M.Fanciulli: Physical Review B, 2001, 64[15], 155201 (7pp)