Structure-composition correlation in niobium containing borophosphate glasses

Glasses of (50-x)P2O5 center dot 20B(2)O(3)center dot 20CaO center dot 10K(2)O center dot xNb(2)O(5) system, with 0 <= x <= 20 mol %, were obtained using the conventional melt quenching method. The correlation between the composition and the structural features was achieved using complementary techniques: X-ray diffraction, differential thermal analysis, infrared and Raman spectroscopy, electron paramagnetic resonance and X-ray photoelectron spectroscopy. The analysis of the experimental data indicates that the oxidation state of niobium ions (Nb3+, Nb4+ or Nb5+) is strongly influenced by Nb2O5/P2O5 ratio which has an important role on the local structure and properties of glasses. In the compositional range of 5-10 mol% Nb2O5, the NB4+ ions are prevalent and they are disposed in sites of octahedral symmetry, acting as a glass modifier. A diminished number of Nb4+ ions occurs at high Nb2O5 concentrations (15 and 20 mol%) leading to Nb4+/(N5+ + Nb3+) (<) 1 ratios, and in this case the Nb5+ ions play a glass former role developing a 3D chain-type lattice by interconnection of differently distorted NbO6 octahedra. The occurrence of highly distorted NbO6 units could impart ferroelectric properties of interest for optoelectronic applications.