EPR and optical studies of Mn2+ ions in Li2O–Na2O–B2O3 glasses – An evidence of mixed alkali effect

EPR and optical absorption spectra of 0.5 mol% MnO2 doped xLi2O–(30 − x)Na2O–69.5B2O3 (5 ⩽ x ⩽ 25) glasses have been studied. The EPR spectra exhibit resonance signals characteristic of Mn2+ ions. The resonance signal at g ≅ 2.0 is due to Mn2+ ions in an environment close to octahedral symmetry, whereas the resonances at g ≅ 4.3 and g ≅ 3.3 are attributed to the rhombic surroundings of the Mn2+ ions. The ionic character (A), the number of spins participating in resonance (N), optical band gap energies (Eopt) and Urbach energies (ΔE) show the mixed alkali effect (MAE) with composition. The present study gives an indication that the size of alkalis we choose, is also an important contributing factor in showing the MAE. The variation of N with temperature obeys the Boltzmann law. The optical absorption spectra show a single broad band at ∼21 000 cm−1 corresponding to the transition 6A1g(S) → 4T1g(G) which exhibits a blue shift with x. The theoretical values of optical basicity (Λth) have also been evaluated.