The structure and properties of xZnO-(67-x)SnO-33P2O5 glasses: (II) Diffraction, NMR, and chromatographic studies

Glasses with nominal molar compositions xZnO-(67-x)SnO-33P2O5 were prepared and their structures were determined by 31P MAS-NMR spectroscopy, HPLC, and by X-ray and neutron diffraction. Compositional dependent changes in the thermal and optical properties described in Part I of this study are attributed to changes in the coordination environments of Sn2+- and Zn2+-ions and their effects on the phosphate anionic network. The 31P MAS-NMR and HPLC data reveal broader distributions of P-tetrahedra in glasses with greater ZnO-contents, due to disproportionation reactions of diphosphate anions in the glass melts to form isolated monophosphate anions and larger triphosphate anions, and due to the loss of some phosphate during glass preparation. The diffraction experiments reveal that Zn2+ prefers tetrahedral coordination, whereas Sn2+ is initially incorporated in isolated, three-coordinated sites that convert to four-coordinated sites with greater ZnO contents. These four-coordinated sites must share corners when incorporated into the glass structure and the transition from glasses dominated by isolated SnO3 pyramids to those with subnetworks of (Sn, Zn)O4 polyhedra occurs near x = 20, and is reflected by breaks in the composition-property trends. The second break in properties occurs near x = 40, and reflects both the dominance of the ZnO4 units in the metal subnetwork, as well an increase in the relative numbers of monophosphate anions in the glass structure.