Synthesis, thermal and spectroscopic characterization of lithium bismuth borate glasses containing mixed transition metal ions.

Glasses with compositions x(2NiO.V₂O₅).(30-x)Li₂O.20Bi₂O₃.50B₂O₃, where x=0, 2, 5 and 7 (LBB, NVLBB1, NVLBB2 and NVLBB3, respectively), 7NiO.23Li₂O.20Bi₂O₃.50B₂O₃ (NLBB) and 7NiO.23Li₂O.20Bi₂O₃.50B₂O₃ (VLBB) are prepared by melt-quenching. The as-prepared samples have been analysed by differential scanning calorimetry (DSC) over the temperature range 200-900°C, infrared spectroscopy (FTIR) over the wave number range 400-1600 cm^-1 and UV-Vis-NIR spectra over the wavelength range 300-2600 nm. The trends of these properties are attributed to changes in the glass network structure. Thermal studies using DSC curves reveal that the glass transition temperature (T_g) of all the glass samples increases with increasing transition metal oxide content in pure lithium bismuth borate glass (LBB). A single peak crystallization temperature (T_x) is observed when individual transition metal ions, i.e. Ni²⁺ or VO²⁺ is added to the base glass (LBB), whereas two crystallization temperatures are obtained when 2NiO.V₂O₅ as a whole is introduced into LBB glass. FTIR studies reveal that Ni²⁺ and VO²⁺ ions lead to structural rearrangements and bismuth exists in [BiO₆] octahedral units only, whereas boroxol rings are absent in the glass network. Optical studies of NLBB and NVLBB glasses indicate that the Ni²⁺ ions occupy only octahedral positions in the glass network, whereas no optical transition was found in the VLBB system. Ligand field theory has been applied for analysis of the optical absorption results. The Racah parameter B (inter-electronic repulsion) and crystal field strength (Δ_oct) are calculated for these transitions in the absorption spectra. Nephelauxetic ratio (β), and optical band gap (E_g) are also evaluated. The infrared spectroscopic results are corroborated by the optical absorption analysis results. [ABSTRACT FROM AUTHOR]