Mixed alkali effect in borate glasses-EPR and optical band gap studies in xNa2O-(30-x)K2O-70 B2O3 glasses doped with Gd3+ions

Mixed alkali borate xNa2O-(30 - x)K2O-70 B2O3 (5 \leq x \leq 25) glasses doped with 0.5 mol% of gadolinium ions have been investigated by using electron paramagnetic resonance (EPR) and optical absorption techniques, as a function of alkali content, to look for the "mixed alkali effect" on the spectral properties of the glasses. The EPR spectrum consists of three prominent features with effective g-values, g \approx 5:6, 2.8 and 2.0, and two weak features at g \approx 3:3 and 4.3. The three EPR signals at g \approx 2:0, g \approx 2:8 and g \approx 5:6 are attributed to Gd3+ ions located at sites with weak, intermediate and strong cubic symmetry fields, respectively. In principle these sites may be of network forming and network modifying types. Ionic radius considerations suggest that gadolinium ions cannot substitute the much smaller boron ions and thus only the network modifier site is acceptable. The number of spins (N) participating in resonance and its paramagnetic susceptibility (\chi) for g \approx 5:6 resonance line have been calculated. It is interesting to note that N and \chi increase with x and reach a maximum around x = 15 and thereafter decrease showing the mixed alkali effect in these glasses. From ultraviolet absorption edges, the optical band gap energies were evaluated. It is interesting to note that the optical band gap energies for these glasses decrease slightly with increasing x and reach a minimum around x = 10, and thereafter increase showing the mixed alkali effect. Optical band gap energies (Eopt) obtained in the present work vary from 2.20-3.35 Ev for both the direct and indirect transitions. The physical parameters of the glasses have been evaluated with respect to the composition