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

Electron paramagnetic resonance (EPR) and optical absorption spectra of VO2 + ions in different alkali calcium borate glasses have been studied. The spin-Hamiltonian parameters (g and A), bonding parameter (PT2) Fermi contact interaction parameter (k) and crystal field parameters have been evaluated. The values of spin-Hamiltonian parameters confirm that vanadyl ions are present in the glasses as VO2 + molecular ions in an octahedral site with a tetragonal compression. At high concentrations of vanadyl ions, the intensity of the EPR resonance signals decreases which has been attributed to the change of oxidation state from V4 + to V5 +. The EPR spectra have also been studied at different temperatures for VO2 + ions in sodium calcium borate glasses. The spin-Hamiltonian parameters are found to be independent of temperature. The magnetic susceptibility chi is calculated from the EPR data. It is observed that as the temperature is decreased the susceptibility increases in accordance with the Curie's law. The theoretical values of optical basicity (A(th)) of glasses have been evaluated. The optical absorption spectrum exhibits two bands characteristic of VO2 + ions in tetragonal symmetry. The two bands have been assigned to the transitions B-2(2g) --> B-2(1g) and B-2(2g) --> E-2(g) in the decreasing order of energy. The octahedral field parameter(Dq) and the tetragonal field parameters Ds and Dt have been evaluated. (C) 2000 Elsevier Science B.V. All rights reserved.