Structure and properties of barium niobophosphate glasses

Phosphate glasses of the ternary system BaO-Nb2O5-P2O5 were studied in two compositional series, namely 40BaO-xNb2O5-(60-x)P2O5 (series A), with x = 0-40 mol% Nb2O5, and yBaO-20Nb2O5-(80-y)P2O5 (series B), with y = 20-60 mol% BaO. Nine homogeneous glassy samples were prepared altogether in both series, and their basic physical properties (density, molar volume, glass transition temperature, dilatometric softening temperature and thermal expansion coefficient) were determined. The chemical durability of the Nb2O5-containing glasses is high and the dissolution rate at 80 °C is lower than 4-5 × 10− 8 g cm− 2 min− 1. The glass transition temperature of the glasses in series A increased significantly with increasing Nb2O5 content from 324 to 727 °C, whereas in the series B with increasing BaO content, Tg increased within the range 620-725 °C. The index of refraction increased more steeply in glass series A with increasing Nb2O5 content than in the glass series B. According to the Raman spectra in the glass series A, at low Nb2O5 content niobium forms isolated octahedra NbO6 incorporated in the glass network, but when Nb2O5 content increases, these octahedra are linked into chains and further into three-dimensional clusters. In glass series B, NbO6 octahedra form only NbONbO chains connecting NbO6 octahedra. NbO6 octahedra clustering is reflected also in the 31P MAS NMR spectra of BaO-Nb2O5-P2O5 glasses showing on a non-monotonous transformation of phosphate units in the direction Q3 → Q2 → Q1 → Q0, both with increasing Nb2O5 content in the first A series and BaO content in the second B series.