Influence of mixed alkali on the spectral properties of vanadyl ions doped $xNa_2O-(30 - x)K_2O-60B_2O_3$ glasses - an EPR and optical study

Electron paramagnetic resonance (EPR) and optical investigations have been performed in the mixed alkali borate $xNa_2O-(30 - x)K_2O-60B_2O_3(5 \le x \le 25)$ glasses doped with 10 mol% of vanadyl ions in order to look for the effect of 'mixed alkalis' on the spectral properties of the glasses. The observed EPR spectra have structures for x > 5 mol% which are characteristic of a hyperfine interaction arising from an unpaired electron with the $^5^1V$ nucleus and it builds up in intensity as x increases. It is observed that the mixed alkali play a significant role in accommodating the vanadyl ions in these mixed alkali glasses and for x > 5 mol%, shows a well resolved hyperfine structure typical for isolated $VO^2^+$ ions. The spin-Hamiltonian parameters (g and A), the dipolar hyperfine coupling parameter (P) and Fermi contact interaction parameter (k) have been evaluated. It is observed that the spin- Hamiltonian parameters do not vary much with the change in composition. It is observed that with increase of x, an increase occurs in tetragonal distortion for $VO^2^+$. The number of spins (N) participating in resonance and the paramagnetic susceptibility (\chi) have been calculated. It is observed that N and \chi increase with x. The optical bandgap energies evaluated for these glasses slightly increase with x and reach a maximum around x = 20 and thereafter decrease showing the mixed alkali effect. Optical band gap energies obtained in the present work vary from 2.73 to 3.10 eV for both the direct and indirect transitions. The physical parameters of the glasses are also determined with respect to the composition