Modifier coordination and phosphate glass networks

The addition of up to ∼16 mol% Cs2O to vitreous P2O5 reduces the glass transition temperature (Tg) by 150 K, whereas further additions up to 60 mol% increases Tg by ∼20 K. 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra indicate that the phosphate network is progressively depolymerized over the entire range of compositions. The property trend is explained by a transition in the Cs+ coordination environment, from isolated Cs-polyhedra below ∼16 mol% Cs2O to a corner-sharing Cs-polyhedral sub-structure in the glasses with greater Cs2O contents. This ‘modifier transition’ does not occur in Al phosphate glasses. 27Al MAS NMR spectra indicate that the average Al coordination number decreases with increasing Al2O3 content to avoid the formation of Al–O–Al bonds in these binary phosphate glasses.