SOLID-STATE HIGH-RESOLUTION AL-27 NUCLEAR MAGNETIC-RESONANCE STUDIES OF THE STRUCTURE OF CAO-AL2O3-P2O5 GLASSES

High field **2**7Al NMR (nuclear magnetic resonance) in combination with magic angle spinning techniques was used to investigate the aluminum coordination in CaO-Al//2O//3-P//2O//5 glasses of varying Al//2O//3 and P//2O//5 contents. In the **2**7Al NMR spectra three resolved lines at minus 21,4, and 37 ppm due to Al nuclei with different shieldings in various electronic environments were observed. The respective lines were assigned to isolated AlO//6 octahedra linked with phosphorus atoms in the second coordination sphere, to some kind of octahedrally coordinated Al atoms partially connected via Al-O-Al bridging bonds, and to AlO//4 tetrahedra incorporated in the phosphate network. Depending on the chemical composition of the glasses the ratio of the line intensities varied markedly. For decreasing P//2O//5/Al//2O//3 ratios an increase in the number of AlO//4 groups was observed, corresponding to an increase in the hydrolytic durability of the glasses.