Allowed and forbidden hyperfine structure of Mn2+ ions in sodium tetraborate glasses-an EPR and optical study

Glass systems of composition (100−x) Na2B4O7·10H2O+x MnSO4·H2O (x=0, 0.1, 0.3, 0.5, 0.75, 1, 1.25, 1.5, 1.75 and 2 mol%) have been studied by using electron paramagnetic resonance (EPR) and optical absorption techniques. The room temperature EPR spectra exhibit resonance signals at g≈2.0 with a sextet hyperfine structure (hfs) besides two weak signals at g≈3.3 and 4.3. The resonance signal at g≅2.0 is due to Mn2+ ions in an environment close to octahedral symmetry, whereas the resonances at g≅3.3 and 4.3 are attributed to the rhombic surroundings of the Mn2+ ions. It is observed that for x=1.25mol% in sodium tetra borate glasses (NTB) the EPR spectrum exhibits a good resolution of allowed as well as forbidden hfs at g≈2.0. The Mn2+ ions in NTB glasses (with A≈95G) are quite ionic in nature. The number of spins (N) participating in resonance at g≈2.0 has been calculated by comparing the area under the absorption curve with that of a standard of known concentration. A linear relationship is observed between log N and 1/T in accordance with the Boltzmann law. Magnetic susceptibilities (χ) were calculated from the EPR spectra at different temperatures. The susceptibility is varying inversely with temperature in accordance with the Curie law, and the Curie constant is evaluated from the 1/χ versus T graph. The zero-field splitting parameter D has also been calculated from the allowed hyperfine (HF) lines. The D value was found to be dependent on temperature. The optical absorption spectrum exhibits a single broad band centered around 470 nm and was assigned to the 6Alg(S)→4Tlg(G) transition. From the ultra violet absorption edges the optical bandgap and Urbach energies have been evaluated.