EPR and optical investigations of manganese ions in alkali lead tetraborate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectra of Mn2+ ions in different alkali lead tetraborate glasses 90R2B4O7 + 9.25PbO + 0.75MnSO4 (R = Li, Na and K) and 90Li2B4O7 + (10 − x)PbO + xMnSO4 (x = 0.1, 0.25, 0.5, 0.75, 1.0, 1.25, 1.5 and 2 mol%) have been studied. The EPR spectrum of all the glass samples exhibit three resonance signals at g = 2.0, 3.3 and 4.3. The resonance signal at g = 2.0 is attributed to the Mn2+ ions in an environment close to an octahedral symmetry. The resonance signals at g = 3.3 and 4.3 have been attributed to the rhombic symmetry of the Mn2+ ions. The effect of temperature (123-433 K) and the composition dependence of EPR signals have been studied for Mn2+ ions in lithium lead tetraborate glasses. It is interesting to observe that the variation of paramagnetic susceptibility (χ) with temperature obeys Curie-Weiss law. From the slope of 1/χ versus T graph, the Curie constant (C) has been evaluated. The zero-field splitting (zfs) parameter D has been calculated for different alkali lead tetraborate glasses from the intensities of the allowed hyperfine lines. The optical absorption spectrum exhibits three bands. An intense and broad band at lower energy side has been assigned to the spin-allowed (5Eg → 5T2g) transition of Mn3+ ions in an octahedral symmetry. The intense and sharp band and a broad band at higher energy side have been assigned to charge transfer bands. A red shift is observed with increase of alkali ion size. The optical band gap energy (Eopt) decreases, whereas the Urbach energies (ΔE) increases with increase of Mn content. The theoretical values of optical basicity (Λth) of the glasses have also been evaluated.