Network Modification in Potassium Borate Glasses:  Structural Studies with NMR and Raman Spectroscopies

Abstract

We have examined the structural changes in the boron−oxygen network that result from chemical modification of glassy boron oxide by potassium oxide. By means of boron-11 dynamic angle spinning (DAS) NMR and Raman spectroscopies, we have identified changes in the structure resulting from conversion of three-coordinate boron to four-coordinate species. We observe coordination changes in two different 3-fold coordinate boron sites: those in six-membered boroxol rings and those in nonring BO3 units. The DAS NMR data show that the fraction of boron in nonring three-coordinate sites decreases as modifier is added. Modification of the boroxol rings is also evident in the Raman data. Assignment of the NMR resonances is made with the help of comparison to spectra of related crystalline potassium borates. Using 11B DAS NMR and 17O magic angle spinning (MAS) NMR, we do not detect significant formation of nonbridging oxygen through 35% added modifier. The changes in the boroxol ring breathing mode in the Raman spectra are attributed to changes in the coordination of the boron that compose the boroxol ring, but coordination changes also occur in the nonring BO3 units. We propose a model that combines these NMR and Raman data, in which modification of both types of 3-fold coordinate sites is included. This model predicts a contribution to the formation of triborate units from nonring BO3 boron, as well as a slightly higher preference for modification of these sites over the boroxol ring boron.

Role of intermediate-range order in predicting the fragility of network-forming liquids near the rigidity transition

“…Zwanziger [35] shown in Fig. 3, reaffirm that the borate structure (at least for x < 33 mol%) can be well accounted for using a collection of just five structural units: loose We believe it is the unique IRO of the borate network that causes its fragility to increase despite the increasing connectivity of the boron atoms.…”

“…Inset illustrates the network structures of the borate system containing BO 3 and BO 4 units. Figure 3 The fraction of boron atoms found in the various IRO superstructures of the borate glass system at selected concentrations of alkali oxide as reported by a recent NMR investigation [35]. These superstructures are illustrated in the inset.…”

“…Together with loose BO 3 (n 3 = 3) and loose BO 4 (n 4 = 4), we can now test the prediction for a coarse grained connectivity given by n IRO = 2f B + 1.5f T + 2f D + 3f 3 + 4f 4 , where the fractions of boron in each structural unit are taken directly from the NMR investigation [35] provided in Fig. 3.…”

“…One prominent feature that distinguishes the borate glass structure from other oxide network-forming glasses is its tendency to form well-documented [28,34,35] intermediate range order (IRO). Both Raman spectroscopy [36,37] and NMR [35,38,39] studies have repeatedly concluded that some fraction of the BO 3 units in B 2 O 3 reside in 6-membered "boroxol" rings (see inset of Fig. 3).…”

Structural Roles of Boron and Silicon in the CaO-SiO2-B2O3 Glasses Using FTIR, Raman, and NMR Spectroscopy

“…Fourth, two bands centered at 1200 and 1430 cm À1 became more apparent with increasing B 2 O 3 content, which originated from the stretching of BO 3 trigonal and BO 4 tetrahedral, respectively. [26,27,29] To quantitatively identify the silicate structure, deconvolution of the Raman spectra was performed in the range of 770 to 1150 cm À1 , which was mainly attributed to the stretching vibration of Q i (Si). Generally, the bands at~870,~960,~990, and~1050 cm À1 could be assigned to Q 0 (Si), Q 1 (Si), Q 2 (Si), and Q 3 (Si), [18][19][20][21] respectively, which were, therefore, employed to deconvolute the Raman curves.…”