Chromium ions in alkali lead borotellurite glasses - An EPR and optical study

Chromium ions doped 90 Na2B4O7 + (9 - x)PbO + 1TeO(2) and 90 K2B4O7 + (9 - x)PbO + 1TeO(2) (x = 3, 4, 5, 6, 7 and 8 mol%) glasses have been studied by using Electron Paramagnetic Resonance (EPR) and optical absorption techniques. The EPR spectra exhibit a broad resonance signal at g = 4.65 and a narrow resonance signal at g = 1.97. The resonance at g = 4.65 can be attributed to isolated Cr3+ ions in rhombic symmetry subjected to strong crystal field effects and the resonance at g = 1.97 is due to Cr5+ ions. The EPR spectra have also been studied at different temperatures (153-393 K). The number of spins (N) participating in the resonance is measured as a function of temperature and the activation energy is calculated. The exchange coupling constant J between chromium ions is calculated. The optical absorption spectrum exhibits bands characteristic of Cr3+ ions in an octahedral symmetry. The optical band gap (E-opt) and Urbach energies (Delta E) have been calculated for all the glass samples and it is observed that Eopt decreases and Delta E increases with increase of chromium content. This has been explained as due to the creation of additional localized states with chromium doping, which overlap and extend in the mobility gap of the glass matrix.