Study of structural modification of sodium aluminophosphate glasses with TiO2 addition through Raman and NMR spectroscopy

Structural modification of sodium aluminophosphate (NAP) glasses with TiO2 addition has been investigated using Raman and MAS-NMR (31P and 27Al) spectroscopy. TiO2 incorporated NAP glasses having composition (mol%): 40Na2O–10Al2O3–xTiO2–(50−x)P2O5 (x=0–20), are prepared by conventional melt quench method. The low-frequency Raman spectrum suggests an increase in the average ionic character of phosphate glass network with addition of TiO2. Raman and 31P MAS-NMR revealed that the glasses without TiO2, consist mainly metaphosphate (Q2) structural units. These are gradually converted into pyrophosphate (Q1) and orthophosphate (Q0) structural units along with the formation of P–O–Ti/P–O–Al linkages. 27Al MAS-NMR revealed the change in coordination of Al from octahedral (AlO6) to tetrahedral (AlO4) for TiO2 above 10 mol%. Raman spectra indicate that TiO2 enters the network in the form of octahedral (TiO6) and tetrahedral (TiO4) structural units and at high concentration of TiO2, tetrahedral structural units are more favourable. Various thermo-physical properties e.g. density (ρ), molar volume (Vm), glass transition temperature (Tg), microhardness (MH), and thermal expansion coefficient (TEC) have been measured as a function of TiO2 content. Variations in the thermo-physical properties are correlated with these structural modifications in the phosphate structural units and consequently changes in the average ionic character of phosphate glass network.