Thermal, structural and crystallization study of Na2O–P2O5–Nb2O5 glasses

Abstract

The present investigation deals with the study of the structure, properties and thermal behaviour of Na2O–P2O5–Nb2O5 glasses prepared in the compositional series 40Na2O-(60-x)P2O5-xNb2O5 (x=0–40 ​mol% Nb2O5). The modification of starting 40Na2O–60P2O5 glass by Nb2O5 revealed changes both in the structure and properties. The glass density steeply increases with increasing Nb2O5 content while the molar volume decreases. Parent sodium ultraphosphate glass was covered with a thin layer of partially hydrolysed glass in air. The chemical durability of Nb2O5 containing glasses was high. The replacement of P2O5 by Nb2O5 resulted in a gradual increase in the refraction index as well as in an increase in glass transition temperature and dilatometric softening temperature. DSC measurements demonstrated that glasses crystallize under heating in the temperature range of 330–880°C. The lowest thermal stability was found for the glass containing 40 ​mol% Nb2O5. The Raman and NMR spectra indicated that the structure of 40Na2O–60P2O5 glass is formed mainly by Q2 and Q1 phosphate units interconnected by P–O–P bonds. The replacement of P2O5 by Nb2O5 leads to depolymerization and the partial transformation of Q2 units into Q1 units and finally to isolated Q0 units. Nb2O5 forms in the glass structure NbO6 octahedra.