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 (V_m), glass transition temperature (T_g), 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.