Modifier coordination and phosphate glass networks

The addition of up to similar to 16 mol% Cs2O to vitreous P2O5 reduces the glass transition temperature (T-g) by 150 K, whereas further additions up t o 60 mol% increases T-g by similar to 20 K. P-31 magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra indicate that the phosphate netwo rk is progressively depolymerized over the entire range of compositions. Th e property trend is explained by a transition in the Cst coordination envir onment, from isolated Cs-polyhedra below similar to 16 mol% Cs2O to a corne r-sharing Cs-polyhedral sub-structure in the glasses with greater Cs2O cont ents. This 'modifier transition' does not occur in Al phosphate glasses. Al -27 MAS NMR spectra indicate that the average Al coordination number decrea ses with increasing Al2O3 content to avoid the formation of Al-O-Al bonds i n these binary phosphate glasses. (C) 2000 Elsevier Science B.V. All rights reserved.