11B NMR studies and structural modeling of Na 2OB 2O 3SiO 2 glasses of high soda content

Abstract

NMR techniques are employed to determine the relative fractions of boron atoms in Na 2OB 2O 3SiO 2 glasses of high soda content. The data show that if enough Na 2O is added, four-coordinated borons are destroyed and borons with one or two non-bridging oxygens are created, but that both the beginning point and the rate of these processes depend strongly on the amount of silica present. These findings are shown to be quantitatively inconsistent with structural models previously suggested in the literature. Utilizing the concept of proportionate atomic sharing of the additional Na 2O, a new structural model is proposed for K </8 ( K = mol% SiO 2/mol% B 2O 3) which is consistent with all the data including previously reported data for glasses in the region of relatively low soda content. Using R = mol% Na 2O/mol% B 2O 3, the new model states that for R MAX = 1 2 + 1 6 K </ R </ R D1 1 2 + 1 4 K , all the additional Na 2O is employed in forming non-bridging oxygens on the silica tetrahedra; then for R D1 </ R </ R D3 = 2 + K, the fraction (K + 1 4 K)/(2 + K) of the additional Na 2O destroys reedmernerite groups and forms pyroborate units plus silica tetrahedra with two non-bridging oxygens per Si atom, while the fraction (2 − 1 4 K)/(2 + K) of the additional Na 2O destroys diborate groups and forms additional pyroborate units.