COORDINATION OF B IN K2O-SIO2-B2O3-P2O5 GLASSES USING B-K-EDGE XANES

High-resolution B K-edge X-ray absorption near-edge structure (XANES) spectra of K2O-SiO2-B2O3-P2O5 glasses are reported using synchrotron r adiation. Two prominent features, peak a at about 194.0 eV and peak b at about 198.0 eV, are observed. On the basis of the qualitative MO di agrams of BO33- and BO54- clusters, peak a is assigned to the transiti on of B 1s electrons to the unoccupied B 2p(z) (pi) states for threef old-coordinated B (B-[3]), and peak b is assigned to the transition of B 1s electrons to the unoccupied B sigma states for fourfold-coordin ated B (B-[4]). B K-edge XANES spectroscopy is established as a method for ''fingerprinting'' B-[3] and B-[4] in berate and borosilicate min erals, glasses, and melts. Also, the relative proportions of B-[4] and B-[3] in the borosilicate glasses are determined from the integrated peak areas for B-[3] and B-[4] edge peaks and are shown to be generall y in good agreement with recent B-11 MAS NMR measurements. However, th e surface and near-surface structure of powder particles of perboric g lasses containing P2O5 appears to have a lower proportion of B-[3] ent ities than the bulk.