NATURE OF ALUMINA IN PHOSPHATE-GLASS .2. STRUCTURE OF SODIUM ALUMINOPHOSPHATE GLASS

We have used Al-27 and P-31 magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS) to describe quantitatively the relationships between the composition, structure, and properties of glasses in the Na2O.Al2O3.P2O5 (NAP) system. In general, the glass properties (evaluated in part I) are most sensitive to changes in Al coordination. Al-27 MAS NMR spectra reveal that octahedrally coordinated Al is most abundant in glasses with O/P ratios less than 3.5, the pyrophosphate structural limit. Tetrahedrally coordinated Al is most abundant in glasses with O/P greater than 3.5. Decreasing Al(OP)6/Al(OP)4 ratio generally correlates with decreasing glass transition temperature and refractive index. The compositional dependence of glass structure and properties can be qualitatively understood using a crystal chemical model based on oxygen charge balancing by the different Al and P moieties.