High-resolution 17O double-rotation NMR characterization of ring and non-ring oxygen in vitreous B2O3

Abstract

The application of double rotation (DOR) NMR to crystalline materials (both inorganic and organic) has made tremendous strides in providing site-specific information about materials in recent years. However ¹⁷O DOR has yet to demonstrate its potential in disordered materials such as glasses. In the present study, we have successfully recorded high resolution ¹⁷O DOR spectra of vitreous B₂O₃ (v-B₂O₃), a highly effective glass-forming oxide of considerable technological importance. Two distinct oxygen sites are resolved and a complete set of ¹⁷O NMR parameters were determined from the DOR spectra. These were assigned to oxygen atoms in the planar boroxol ring [B₃O₆] and in the non-boroxol [BO₃] groups which share oxygen with the ring boron atoms. This assignment was based on the similarity of all of their ¹⁷O parameters with those found by DFT calculation for caesium enneaborate, Cs₂O·9B₂O₃, which has two boroxol rings in its structure. The boroxol ring oxygens have a more positive chemical shift, a larger shift anisotropy and a smaller electric field gradient than non ring oxygens (O_R: δ_iso = 100 ± 1 ppm, span = 180 ± 20 ppm, skew = −0.4 ± 0.1, P_q = 5.0 ± 0.2 MHz; O_NR: δ_iso = 86 ± 1 ppm, span = 100 ± 20 ppm, skew = 0.1 ± 0.1, P_q = 5.7 ± 0.2 MHz). The relative proportions of the two sites in v-B₂O₃ are ∼1 : 1, as expected if all three boron atoms in the boroxol ring are each connected to one oxygen in a linking [BO₃] group and there are very few [BO₃]–[BO₃] linkages. We see no evidence for a third oxygen site such as has been reported in an earlier study of v-B₂O₃. This work demonstrates the potential of ¹⁷O DOR to provide site-specific information in disordered materials.