ABSTRACT

Borate crystals and glasses are an important class of materials that have received much attention because of their wide-ranging importance in technological processes, including extensive use in the areas of optics, display, and telecommunication [1-3]. The fundamental understanding of these materials and formulation of accurate predictive models for the compositional dependence of physicochemical properties require detailed knowledge of their atomic scale structure and the dynamical phenomena they exhibit, including transport and relaxation. High-resolution 11B nuclear magnetic resonance (NMR) spectroscopy has played an important role in elucidating the shortrange structure around B atoms in a wide variety of borate crystals and glasses [4-10]. The natural abundance of the 11B isotope is ~80% and it is a highly NMR-sensitive quadrupolar (nuclear spin I = 3/2) nuclide with a relatively large gyromagnetic ratio. Boron atoms in borates are either three-or fourfold coordinated to oxygen atoms, forming BO3 planar triangles or BO4 tetrahedra, respectively. The B sites in planar BO3 triangles are characterized approximately by C3 symmetry and, consequently, by an NMR line shape dominated by a relatively large quadrupolar coupling constant CQ (2.4 ≤ CQ ≤ 2.9 MHz). On the other hand, the B sites in the BO4 tetrahedra are characterized by higher symmetry than those in BO3 triangles. Consequently, the NMR line shapes of BO4 sites are nearly Gaussian, resulting from small CQ values (typically 0.0 ≤ CQ ≤ 0.5 MHz). Moreover, the 11B isotropic chemical shifts δiso of the BO3 and BO4 sites differ by ~15 ppm to 20 ppm [11]. These differences in CQ and

19.1 Introduction .................................................................................................. 293 19.2 Calculation Methodology ............................................................................. 295 19.3 Results and Discussion ................................................................................. 295 19.4 Conclusions ...................................................................................................300 References .............................................................................................................. 301

δiso of the BO3 and BO4 sites make them easily identižable in the 11B NMR spectra. Although δiso for each of these sites vary over a small range, recent detailed 11B NMR studies of a wide variety of borate crystals have shown that at least in the case of BO3 sites, such variation can be empirically associated with the existence of structural systematics such as the number of bridging vs. nonbridging oxygen nearest neighbors and the sum of cation-oxygen bond strengths [11]. Recently available high-želd 11B NMR data have also shown the presence of chemical shift anisotropy (CSA) in BO3 and BO4 sites, although precise experimental determinations of CSA and asymmetry parameter ηCS of the CSA remain experimentally challenging [11-13].