Structure and properties of barium niobophosphate glasses.

Phosphate glasses of the ternary system BaO-Nb 2 O 5 -P 2 O 5 were studied in two compositional series, namely 40BaO-xNb 2 O 5 -(60-x)P 2 O 5 (series A), with x = 0–40 mol% Nb 2 O 5 , and yBaO-20Nb 2 O 5 -(80-y)P 2 O 5 (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 Nb 2 O 5 -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 Nb 2 O 5 content from 324 to 727 °C, whereas in the series B with increasing BaO content, T g increased within the range 620–725 °C. The index of refraction increased more steeply in glass series A with increasing Nb 2 O 5 content than in the glass series B. According to the Raman spectra in the glass series A, at low Nb 2 O 5 content niobium forms isolated octahedra NbO 6 incorporated in the glass network, but when Nb 2 O 5 content increases, these octahedra are linked into chains and further into three-dimensional clusters. In glass series B, NbO 6 octahedra form only Nb O Nb O chains connecting NbO 6 octahedra. NbO 6 octahedra clustering is reflected also in the 31 P MAS NMR spectra of BaO-Nb 2 O 5 -P 2 O 5 glasses showing on a non-monotonous transformation of phosphate units in the direction Q 3 → Q 2 → Q 1 → Q 0 , both with increasing Nb 2 O 5 content in the first A series and BaO content in the second B series. [ABSTRACT FROM AUTHOR]