Optical and electron paramagnetic resonance studies of VO2+ doped mixed alkali–alkaline earth oxide borate glass system

Glasses with the composition xK2O–(25−x) Li2O–12·5BaO–12·5MgO–48B2O3–2V2O5 (0≤x≤20) were prepared by the melt quenching method. The mixed alkali effect (MAE) has been investigated in the glass system through density, molar volume, optical absorption and EPR studies. VO2+ ions were used as the spin probes. From the optical absorption studies the values of the optical band gap (Eopt) for indirect transitions and the Urbarch energy (∆E) were evaluated. The values of Eopt, ∆E and density exhibited nonlinear behaviour with the variation of Li2O content, which may be attributed to the mixed alkali effect. The EPR spectra have structures which are characteristic of a hyperfine interaction arising from an unpaired electron with the 51V nucleus and it builds up in intensity as x increases. The spin-Hamiltonian parameters were evaluated from the EPR spectra. For x=0 mol%, the EPR spectrum was found to be more intense and well resolved. The variation of g‖ versus A‖ and N versus 1/x as a function of Li2O content was found to be nonlinear, which may be due to the mixed alkali effect. The results obtained were explained.