Influence of ligand coordination of cobalt ions on structural properties of ZnO-ZnF2-B2O3 glass system by means of spectroscopic studies

The glasses of the composition 10ZnO-30ZnF₂-60B₂O₃ doped with different concentrations of CoO were prepared. Differential scanning calorimetric (DSC) studies, optical absorption, photoluminescence and infrared spectra of these glasses have been carried out. DSC studies have indicated that the resistance of the glass against devitrification increases with the increase in the concentration of CoO. Optical absorption spectra have exhibited one octahedral band due to ⁴T_1g(F)→²T_1g(H) and two tetrahedral bands due to ⁴A₂(⁴F)→⁴T₁(⁴P) ⁴A₂(⁴F)→⁴T₁(⁴F) transitions of Co²⁺ ions at about 525, 570 and 1400 nm, respectively. As the concentration of CoO is increased the tetrahedral bands are observed to grow at the expense of octahedral band. The luminescence spectra have exhibited two emission bands in the spectral regions of 600-700 nm and 800-900 nm due to ⁴T₁(⁴P)→⁴A₂(⁴F) and ⁴T₁(⁴P)→⁴T₂(⁴F) tetrahedral transitions of Co²⁺ ions, respectively. With the increasing content of cobalt ions in the glass matrix, the half width and intensity of these bands are observed to increase. The analysis of the results of these two spectra coupled with IR spectra has indicated that as the concentration of CoO is increased in the glass matrix, the tetrahedral occupancy of cobalt ions dominates over the octahedral occupancy and increase the rigidity of the glass network.