The effect of mixed alkali on EPR and optical absorption spectra in mixed alkali borate xNa2O–(30 − x)K2O–70B2O3 glasses doped with iron ions

Electron paramagnetic resonance (EPR) and optical absorption studies of iron doped mixed alkali borate glasses, xNa2O–(30 − x)K2O–70B2O3 (5 ⩽ x ⩽ 25) have been investigated as a function of alkali content to look for the ‘mixed alkali effect’ on the spectral properties of the glasses. The EPR spectra of all the investigated samples exhibit resonance signals which are characteristic of the Fe3+ ions. The EPR spectrum exhibits an intense resonance signal at g = 4.02 ± 0.1, a moderately intense signal at g = 2.02 ± 0.1 and a shoulder in the region of g = 7.20 ± 0.5. The existence of the resonances at g = 4.02 and g = 7.20 have been attributed to Fe3+ ions in rhombic and axial symmetry sites respectively. The g = 2.02 resonance is due to Fe3+ ions coupled by exchange interactions. The number of spins (N) participating in resonance and its paramagnetic susceptibility (χ) have been evaluated. It is observed that N and χ decrease with x up to x = 10 and then increase up to x = 15 exhibiting a minimum at x = 10 and a maximum at x = 15 and thereafter it gradually decreases. The optical absorption spectrum exhibits three bands assigned to 6A1g(S) → 4A1g(G), 4Eg(G); 6A1g(S) → 4T2g(G) of Fe3+ and the other band is assigned to Fe2+–Fe3+ inter-valence charge transfer band. From ultraviolet absorption edges, the optical bandgap and Urbach energies have been evaluated. The optical band gap energy obtained in the present work varies 2.74–3.77 eV for both the direct and indirect transitions. The physical parameters of all the glasses have been evaluated with respect to x.