A study of electron paramagnetic resonance and optical absorption spectra of VO2+ ions in alkali barium phosphate glasses

The electron paramagnetic resonance and optical absorption spectra of VO2+ ions in different alkali barium phosphate glasses have been studied and the results have been analysed using C-4v symmetry. The EPR spectrum shows eight hyperfine lines characteristic of VO2+ ions. From the observed EPR spectra, the spin-Hamiltonian parameters, Fermi contact interaction parameter k, dipolar hyperfine coupling parameter P have been evaluated. The EPR spectra exhibit a marked concentration dependence. The EPR spectra were also recorded at different temperatures. The number of spins participating in resonance is found to be increasing with decreasing temperature. The paramagnetic susceptibility X is calculated from the EPR data. As the temperature decreases, X increases in accordance with Curie's law. The optical absorption spectrum exhibits two bands corresponding to the transitions B-2(2g) --> B-2(1g) and B-2(2g) --> E-2(g), characteristic of VO2+ ions in octahedral symmetry with tetragonal distortion. From the ultraviolet edges of these optical absorption spectra, the optical band gap energies (E-opt) of the glass samples have been calculated. The optical band gap energy varies with alkali content. By correlating EPR and optical absorption data, the molecular orbital coefficients have been evaluated. The theoretical values of optical basicity of the glasses have also been evaluated.