Correlations between EPR and optical properties: Diborovanadate glasses in a highly modified regime

Abstract

Glasses with composition x V2O5 - 10PbO – (90−x) NaBO2 with 10≤x≤30 were prepared by a novel, energy-efficient and economical microwave method. The Electron Paramagnetic Resonance (EPR) spectra of all the glasses investigated exhibited a well-defined hyperfine structure, from which the spin Hamiltonian parameters of the VO2+ ion, dipolar hyperfine coupling parameters, and the fermi-contact interaction term (K) have been calculated. The values of g-tensor reveal the existence of the VO2+ ion in octahedral coordination with a tetragonally compressed site. The variations seen in P, K, Pβ2K and PK/γ with V2O5 content indicate the s character of the magnetic spin of vanadium which arises due to the partial unpairing of s-electrons. Also, a decrease in K with V2O5 mol% suggests a strong VO interaction, makes the bond length (VO) in the vanadyl group shorter and distorts the vanadyl group from Oh to C4V. The EPR and UV–Vis parameters are analyzed on the basis of structural motifs formed due to the network modification. The molecular orbital coefficients α2 and γ2 have been calculated. The ratio, 1−γ21−α2 increases with increasing V2O5 mol%, suggesting that the σ-bonding between the V atom and the equatorial ligand is weakening. IR studies reveal the presence of [BO3/2]0, [BO4/2]− and [B2V2O9]2− structural groups in the glass matrix.