Mixed Alkali Effect in Li2O -Na2O-B2O3 Glasses Containing CuO-An EPR and Optical Study

The “mixed alkali effect” is one of the classical anomalies of glass science. We report here an interesting phenomenon “Mixed Alkali Effect” observed for a series of Cu ions doped xLi2O-(30x)Na2O-70B2O3 (5 ≤ x ≤ 25) glasses, investigated by EPR and optical absorption techniques. The EPR spectra of all the investigated samples exhibit resonance signals characteristic of Cu ions in octahedral sites with tetragonal distortion. It is found that the spin-Hamiltonian parameters do not vary much with x. However, the number of spins participating in resonance (N) and the paramagnetic susceptibility (χ) exhibits the mixed alkali effect. The N is also measured as a function of temperature and the activation energy is evaluated. It is observed that the temperature dependence of paramagnetic susceptibility (χ) obeys Curie-Weiss law. The optical absorption spectra exhibit a strong band corresponding to the transition (B1g → B 2g) centered at 13280 cm . By correlating the EPR and optical absorption data, the molecular orbital coefficients α and β are evaluated. It is interesting to observe that the optical band gap and Urbach energies show the MAE.