Multinuclear MAS-NMR studies of Al2O3-B2O3-P2O5 glasses

Solid state B-11, Al-27 and P-31 nuclear magnetic resonance spectroscopy has been used to study the microstructure of 16 Al2O3-B2O3-P2O5 glasses whose constitutents may all act as glass formers. To obtain the fundamental parameters for the quadrupolar Al-27 nuclei satellite transition spectroscopy has been applied. As a result, boron occurs always four-coordinated, while aluminium atoms are distinguished by three well defined environments assigned to four-, five- and six-coordination; the proportion of four-coordinated Al species decreases with increasing P2O5 content, that of five- and six-coordinated species increases. Phosphorus is tetrahedrally coordinated, its chemical shift varies systematically as a function of the bonding behavior in the second coordination sphere. The experimental findings can be described by a random network model for the whole glass-forming region, which includes a continuous transition from a silica-like structure of AlPO4 and BPO4 for [P2O5]/([B2O3] + [Al2O3(4)]) ratios of about 1, to a more loosened structure containing AlO4, AlO5 as well as Al(PO3)(3) elements, if there is excess oxygen and this ratio becomes larger.