EPR and optical studies of Mn 2+ ions in Li 2O–Na 2O–B 2O 3 glasses – An evidence of mixed alkali effect

Abstract

EPR and optical absorption spectra of 0.5 mol% MnO 2 doped xLi 2O–(30 − x)Na 2O–69.5B 2O 3 (5 ⩽ x ⩽ 25) glasses have been studied. The EPR spectra exhibit resonance signals characteristic of Mn 2+ ions. The resonance signal at g ≅ 2.0 is due to Mn 2+ 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 Mn 2+ ions. The ionic character ( A), the number of spins participating in resonance ( N), optical band gap energies ( E opt) and Urbach energies (Δ E) 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 6A 1g(S) → 4T 1g(G) which exhibits a blue shift with x. The theoretical values of optical basicity ( Λ th) have also been evaluated.