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

The present investigation deals with the study of the structure, properties and thermal behaviour of Na2O-P2O5-Nb2O5 glasses prepared in the compositional series 40Na(2)O-(60-x)P2O5-xNb(2)O(5) (x=0-40 mol% Nb2O5). The modification of starting 40Na(2)O-60P(2)O(5) 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 degrees C. The lowest thermal stability was found for the glass containing 40 mol% Nb2O5. The Raman and NMR spectra indicated that the structure of 40Na(2)O-60P(2)O(5) glass is formed mainly by Q(2) and Q(1) phosphate units interconnected by P-O-P bonds. The replacement of P2O5 by Nb2O5 leads to depolymerization and the partial transformation of Q(2) units into Q(1) units and finally to isolated Q(0) units. Nb2O5 forms in the glass structure NbO6 octahedra.