Properties of potassium niobium germanate glasses

Recent studies of potassium niobium germanate glasses have concentrated on the production of transparent glass-ceramics for nonlinear optical devices. Since very little is known about the properties of these glasses and their dependence on composition, the density, molar volume, glass transformation temperature (Tg) and electrical conductivity have been determined for two series of potassium niobium germanate glasses lying on lines of constant Nb2O5 to K2O concentrations. Replacement of GeO2 by Nb2O5 has little effect on the location of anomalies in the density and Tg, but the anomaly in the molar volume disappears. Niobium oxide decreases the density of these glasses at low concentrations, but increases the density at higher concentrations. The Tg increases by 50 to 100 K as Nb2O5 replaces GeO2. The electrical conductivity decreases at all K2O concentrations as GeO2 is replaced by Nb2O5. A structural model is proposed where the Ge-O-Ge network is progressively replaced by a Nb-O-Nb network with increasing Nb2O5 content.