Structural influence of mixed transition metal ions on lithium bismuth borate glasses

Lithium bismuth borate glasses containing mixed transition metals having composition 7CoO center dot 23Li(2)O center dot 20Bi(2)O(3)center dot 50B(2)O(3) (CLBB), 7V(2)O(5)center dot 23Li(2)O center dot 20Bi(2)O(3)center dot 50B(2)O(3) (VLBB) and x(2CoO center dot V2O5)center dot(30 - x) Li2O center dot 20Bi(2)O(3)center dot 50B(2)O(3) (x = 0.0 (LBB) and x = 2.0, 5.0, 7.0, 10.0 mol% (CVLBB1-4)) are synthesized via melt quench route. The synthesized compositions are investigated for their physical properties using density (D) and molar volume (V-m), thermal properties by analyzing DSC/TG thermo-graphs, structural properties using IR absorption spectra in the mid-IR range and optical properties using UV-Vis-NIR spectroscopy. The Electron Paramagnetic Resonance (EPR) spectra of vanadyl and cobalt ion have been analyzed to study compositional effects on spin-Hamiltonian parameters. The non linear variations in physical properties depict a strong structural influence of Co/V- oxides on the glassy matrix. The compositional variations in characteristic temperatures (glass transition temperature T-g, glass crystallization temperature T-p and glass melting temperature T-m) reveals that T-g for glass samples CLBB is relatively less than that of pure lithium bismuth borate (LBB) glass sample wherein T-g for sample VLBB is higher than that of LBB. The increase in T-g (as compared with LBB) with an enhanced substitution of mixed transition metal oxides (2CoO center dot V2O5) shows a progressive structure modification of bismuth borate matrix. These predictions are very well corroborated by corresponding compositional trends of T-p and T-m. FTIR studies reveal that Co2+& VO2+ ions lead to structural rearrangements through the conversion of three-coordinated boron into four coordinated boron and thereby reducing number of non-bridging oxygen atoms. Bismuth is found to exist in [BiO6] octahedral units only, whereas boroxol rings are not present in the glass network. The theoretical values of optical basicity (Lambda(th)) and corresponding oxide ion polarizability (alpha(2-)(o)) have also been calculated to investigate oxygen covalency of glass matrix. Trends in both these parameters suggested an increase in ionic bonding on substitution of divalent transition metal cations causing a more bonding compaction in glass structure. The UV-Vis-NIR spectra suggest that cobalt ions exist as Co2+ states in octahedral coordination in glass network. Inter-electronic repulsion parameter and crystal field splitting energy were evaluated to understand the site symmetry around Co2+- ion in glass. X-band EPR spectra suggest that vanadium ions (V4+) exists as VO2+- ions in octahedral coordination with tetragonal compression. Spin Hamiltonian parameters g- values and A-values of VO2+ ions in glass were calculated. For sample CLBB two resonance lines in EPR spectrum attribute to octahedral symmetry around Co2+- ions were observed. (C) 2017 Elsevier Masson SAS. All rights reserved.