EPR and optical studies of microwave synthesized glasses containing VO2+ ions: Meta and pyrophosphate regime

Glasses with compositions xV2O5 - (100 - x)[0.8NaPO3 - 0.2ZnO] (where 0.2 ≤ x ≤ 10 in mol%) were prepared by a new microwave heating method. The Electron Paramagnetic Resoannce (EPR) spectra of vanadyl ions in the glasses exhibit a hyperfine structure. From EPR spectra the spin Hamiltonian parameters g∥, g⊥, A∥, A⊥, dipolar hyperfine coupling parameter (P), Fermi contact interaction parameter (K), P K and PK/γ have been evaluated. An increase in the ratio of Δg‖/Δg⊥ with an increase in V2O5 content shows that the tetragonal nature of the V⁴⁺O6 complex is enhanced. The increase in A'|| and P indicates that the 3dxy orbit contracts with V2O5 content. The molecular orbital coefficients α², γ² have been determined by correlating the EPR and optical absorption data. The values of α² increase while γ² decreases with an increase in V2O5 content, indicating the σ - bonding between the vanadium atom and the equatorial ligands decreases and the effect of π - bonding with the vanadyl oxygen increases. The bond ionicity and covalency of the glass network has been calculated using Pauling's electronegativity support the experimental findings. Observed variations in the optical band gap, refractive index, molar refraction, metallization criterion, electronic polarizability of the oxide ion and optical basicity are correlated with the glass structure. The photoluminescence (PL) excitation spectra reveal two individual peaks at 338 and 468 nm which are ascribed to the charge transfer reaction in [VO4]³⁻ groups. An emission peak occurs at 531 nm and concentration quenching is seen beyond 0.5 mol% of V2O5