Non-random cation distribution in sodium–strontium–phosphate glasses

Abstract

Vitreous compositions in the (0.55− x)Na 2O: xSrO:0.45P 2O 5 (0⩽ x⩽0.55) system were characterized by differential scanning calorimetry and 31P solid state NMR. High strontium containing glasses were found to be partly crystallized. In the pure glass samples a general increase in T g and a decrease in isotropic chemical shift with increasing x were observed. Two distinct linear ranges were observed in plots of these parameters against composition, with a transition point at x≈0.20. This composition corresponds to the point at which all Na + ions associated with charge balance of the terminal Q 1 phosphate tetrahedra are substituted for Sr 2+. In the mixed cation glasses, this suggests a non-random distribution of cations, with preferential location of Sr 2+ ions near the chain ends. Crystalline models have been used to discuss trends in the variation of chemical shift anisotropy and propose possible coordination environments for the metal cations in the glasses.