Single-crystal EPR and DFT studies of a [BO₄]0 center in datolite: electronic structure, formation mechanism and implications

摘要

A natural datolite CaBSiO₄(OH) (Bergen Hill, NJ, USA), before and after gamma-ray irradiation (up to ~70 kGy), has been investigated by single-crystal and powder electron paramagnetic resonance (EPR) spectroscopy from 10 to 295 K. EPR spectra of gamma-ray-irradiated datolite show the presence of a boron-associated oxygen hole center (BOHC) and an atomic hydrogen center (H0), both of which grow with increasing radiation dose. The principal g and A(11B) values of the BOHC at 10 K are: g ₁ = 2.04817(3), g ₂ = 2.01179(2), g ₃ = 2.00310(2), A ₁ = −0.401(7) mT, A ₂ = −0.906(2) mT, A ₃ = −0.985(2) mT, with the orientations of the g ₁ and A ₁ axes approximately along the B–OH bond direction. These experimental results suggest that the BOHC represents hole trapping on the hydroxyl oxygen atom after the removal of the proton (i.e. a [BO₄]0 center): via a reaction O₃BOH → O₃BO· + H0, where · denotes the unpaired electron. Density functional theory (DFT) calculations (CRYSTAL06, B3PW, all-electron basis sets, and 1 × 2 × 2 supercell) support the proposed structural model and yield the following 11B hyperfine coupling constants: A ₁ = −0.429 mT, A ₂ = −0.901 mT, A ₃ = −0.954 mT, in excellent agreement with the experimental results. The [BO₄]0 center undergoes the onset of thermal decay at ~200°C and is completely annealed out at 375°C but can be restored readily by gamma-ray irradiation. Isothermal annealing experiments show that the [BO₄]0 center exhibits a second-order thermal decay with an activation energy of 0.96 eV. The confirmation of the [BO₄]0 center (and its formation from the O₃BOH precursor) in datolite has implications for not only understanding of BOHCs in alkali borosilicate glasses but also their applications to nuclear waste disposal.