Role of titanium in sodium titanophosphate glasses and a model of structural units

Glasses with composition 50Na2O–xTiO2–(50−x)P2O5 (0≤x≤15) have been studied. Their density, molar volume, glass transition temperature and infrared spectroscopy have been investigated. The evolution of infrared spectra with the different constituents is studied and the oxide-forming character of TiO2 is discussed. In sodium titanophosphate glasses, the structures consist of distorted Ti octahedra linked to metaphosphate and pyrophosphate units through P–O–Ti bonds. The metaphosphate (P2O62−) and pyrophosphate (P2O74−) units coexist. Metaphosphate groups are the predominant structural units in glasses with the lowest TiO2 content. For TiO2-rich glasses, the pyrophosphate groups become the predominant structural units. A structural model is proposed to explain the experimental results. According to this model TiO2 is assumed to be present as six coordinated octahedral [TiO6/2]2− units, again with shared corners.