EPR and Optical Studies of Vanady Ions in Alkali Lead Borotellurite Glasses

Electron Paramagnetic Resonance (EPR) and optical absorption spectra of $VO^{2+}$ ions in different alkali lead borotellurite glasses have been studied. The spin-Hamiltonian parameters (g and A), bonding parameter $\beta_{2}.^{2}$ and Fermi contact interaction parameter k have been calculated. The values of spin-Hamiltonian parameters confirm that the vanadyl ions are present in the glasses as $VO^{2+}$ molecular ions in an octahedral site with a tetragonal compression. The number of spins (N) participating in resonance is calculated as a function of temperature (123-393 K) for 9 mol% of $VO^{2+}$ ions in lithium lead borotellurite glass sample. It is observed that N obeys the Boltzmann law. From EPR data, the paramagnetic susceptibility (\chi) is calculated at various temperatures and the Curie constant has been evaluated from $1/\chi$ graph. The optical absorption spectrum exhibits two bands characteristic of $VO^{2+}$ ions in tetragonal symmetry. The band gap $(E_{opt})$ and the Urbach energies $(\triangle E)$ have been determined from the ultraviolet absorption edges and are found to be dependant on the size of the alkali ion. The theoretical values of optical basicity $(A_{th})$ of these glasses have also been evaluated.