Formation and properties of sodium tantalum silicate glasses

A large glass forming region exists in the sodium tantalum silicate system. Addition of Ta2O5 to sodium silicate glasses results in a large increase in density, refractive index, and glass transformation temperature and a decrease in the thermal expansion coefficient. Properties of sodium tantalum silicate glasses are compared with those of the corresponding sodium niobium silicate glasses, which exhibit similar property/composition trends. A speculative model, which assumes that Ta5+ ions are octahedrally coordinated by oxygens, is proposed for the structure of these glasses.