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 (B-[3]). The anomalously high divergence of tetrahedral (B-[4]-O) bond lengths in HBO2(II) and sinhalite is used to resolve fine structure at the B-[4] K edge due to splitting of sigma*(t(2)) antibonding orbitals. For HBO2(II), XANES peaks at 196.9 and 199.3 eV are assigned to B-[4]-O distances of 1.564 and similar to1.440 (x3) Angstrom, respectively, and, for sinhalite, peaks at 196.8, 197.9, and 199.6 eV are assigned to distances of 1.586, 1.483 (x2), and 1.442 Angstrom, respectively. A correlation between peak splitting at the B-[4] 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 <Angstrom>, show that almost all B-[4] in HBO2(II) and about one-third of the B-[4] in sinhalite are converted to B-[3] 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.