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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.

Synchrotron radiation boron K-edge XANES spectra collected in fluorescence yield mode are reported for monoclinic metaboric acid [HBO2(II)], sinhalite (MgAlBO4), and a selection of boron oxides in which B is exclusively in trigonal coordination ([3]B). The anomalously high divergence of tetrahedral ([4]B–O) bond lengths in HBO2(II) and sinhalite is used to resolve fine structure at the [4]B K edge due to splitting of σ*(t2) antibonding orbitals. For HBO2(II), XANES peaks at 196.9 and 199.3 eV are assigned to [4]B–O distances of 1.564 and ∼1.440 (×3) A, respectively, and, for sinhalite, peaks at 196.8, 197.9, and 199.6 eV are assigned to distances of 1.586, 1.483 (×2), and 1.442 A, respectively. A correlation between peak splitting at the [4]B K edge and divergence of tetrahedral bond length is established for borates and borosilicates using data for sinhalite, HBO2(II), ferroaxinite, danburite, datolite, and BPO4. B K-edge XANES spectra collected in total electron yield mode, which probes to