Boron K-edge XANES of boron oxides: tetrahedral B–O distances and near-surface alteration

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) Å, 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?Å, 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 <60?Å, show that almost all ^4]B in HBO₂(II) and about one-third of the ^4]B in sinhalite are converted to ^3]B in the near-surface structure. Moreover, HBO₂(II), HBO₂(III), sassolite (boric acid; H₃BO₃), and v-B₂O₃, which have markedly different bulk structures, have a similar near-surface layer composed of a relaxed anhydrous network of BO₃ groups.