Coordination state of Nb5+ ions in silicate and gallate glasses as studied by Raman spectroscopy

Raman spectra of K2O-Nb2O5-SiO2 glasses are measured in order to compare the coordination state of Nb5+ ions in gallate glasses with that in silicate glasses. It is found that less-distorted NbO6 octahedra with no non-bridging oxygens as well as NbO6 octahedra with non-bridging oxygens and/or with much distortion are present in the K2O-Nb2O5-SiO2 glasses. The Raman band in the 800 to 900 cm−1 region is attributed to the NbO6 octahedra with non-bridging oxygens and/or with much distortion. The broad bands in the 600 to 800 cm−1 region are attributed to less-distorted NbO6 octahedra with no non-bridging oxygens. An increase in the molar ratio Nb2O5/K2O leads to an increase in the oxygens shared by more than two polyhedra and/or a decrease in non-bridging oxygens for the NbO6 octahedra which possess non-bridging oxygens, or to an increase of distortion for much-distorted NbO6 octahedra. At the same time, an increase in the molar ratio Nb2O5/K2O increases the less-distorted NbO6 octahedra with no non-bridging oxygens. In short, GaO4 tetrahedra and NbO6 octahedra compete to attract alkali ions in gallate glasses but such competition is not found in silicate glasses.