Network structures and the properties of Na-Ca-Sr-borophosphate glasses

Borophosphate glasses were prepared with the nominal molar compositions 16Na2O-(24-y)CaO-ySrO-xB2O3-(60-x)P2O5 (mol%), where 0<x<60 and y=0, 12, and 24. Information about the compositional dependence of borate and phosphate site speciation and next nearest neighbor linkages was obtained by 11B and 31P MAS NMR and Raman spectroscopies, and by high pressure liquid chromatography (HPLC). With the initial replacement of P2O5 by B2O3, tetrahedral borate sites linked to four phosphate anions, B(oP)4, are created in the glass structure, and the average phosphate anion becomes smaller as bridging PoP bonds are replaced by bridging PoB bonds. With further increases in the B2O3 content, borate units, including B-triangles, replace phosphate units linked to the B -tetrahedra. Compositional trends for the glass transition temperature (Tg) and molar volume are explained by considering the number and types of bridging oxygens per glass former, consistent with topological models reported elsewhere.