Local structure of sodium aluminum metaphosphate glasses

High-resolution solid-state P-31, Al-27, and Na-23 nuclear magnetic resonance (NMR), Raman spectroscopy, differential scanning calorimetry, and density measurements were used to characterize the local structure of the glasses (1-x)(NaPO3)(3) . xAl(PO3)(3). A systematic increase in density, glass transition temperature (T-g), and frequencies of P-O stretching vibrations for terminal nonbridging oxygens (NBOs) was observed as the Al content was increased. A change of slope in the behavior of Tg as a function of x was clearly detected around x = 0.25 +/- 0.03. Also, changes of behavior in the Na-23 NMR line shape and in the Raman band of the PO2 symmetric vibration are detected for concentrations higher than x = 0.17. According to these facts, a reorganization of the network affecting the degree of connectivity between phosphate chains is proposed to explain the observed behaviors of T-g and P-NBO vibrations in these glasses.