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.