Characterization of borate glasses by W-band pulse electron-nuclear double resonance spectroscopy

(100−x)mol% B2O3 x mol % Me2O (Me=Li,Na,K) glasses, exposed to γ-Co60 irradiation to produce paramagnetic states, were characterized by W-band (95GHz) pulse electron-nuclear double resonance (ENDOR) spectroscopy in order to characterize local structures occurring in the range of compositions between x=16 and x=25 at which the “boron oxide” anomaly occurs. The high resolution of nuclear frequencies allowed resolving the Li7 and B11 ENDOR lines. In the samples with x=16 and x=20 glasses, B11 hyperfine couplings of 16, 24, and 36MHz were observed and attributed to the tetraborate, triborate, and boron oxygen hole center (BOHC) structures, respectively. The x=25 samples showed hyperfine couplings of 15MHz for the tetraborate and 36MHz for BOHC. Density functional theory (DFT) calculations predicted for these structures negative hyperfine couplings, which were confirmed by W-band ENDOR. This suggests that a spin polarization mechanism accounts for the negative hyperfine structure splitting.