Mixed alkali effect in Li2O–Na2O–B2O3 glasses containing CuO – An EPR and optical study

Electron Paramagnetic Resonance (EPR) and optical absorption spectra of 0.5 mol% CuO doped xLi2O–(30 − x)Na2O–69.5B2O3 (5 ⩽ x ⩽ 25) mixed alkali glasses have been investigated. The EPR spectra of all the investigated samples exhibit resonance signals characteristic of Cu2+ ions in octahedral sites with tetragonal distortion. It is found that the spin-Hamiltonian parameters do not vary much with x. It is interesting to observe that the number of Cu2+ ions participating in resonance (N) and its paramagnetic susceptibility (χ) exhibits the mixed alkali effect with composition. It is observed that the temperature dependence of paramagnetic susceptibility (χ) obeys Curie–Weiss law. The paramagnetic Curie temperature (θp) is negative for the investigated sample, which suggests that the copper ions are antiferromagnetically coupled by negative super exchange interactions at very low temperatures. A broad band corresponding to the transition 2B1g → 2B2g in the optical absorption spectrum shows a blue shift with x. By correlating the EPR and optical absorption data, the molecular orbital coefficients α2 and β12 have been evaluated. It is interesting to observe that the optical band gap (Eopt) and Urbach energies (ΔE) exhibit the mixed alkali effect. The theoretical values of optical basicity (Λth) have also been evaluated.