Cu2+ ions in sodium fluoride-sodium borate glasses studied by EPR and optical absorption techniques

Electron paramagnetic resonance (EPR) and optical absorption spectra of Cu2+ ions in 80Na2B4O7-(20 − x)NaF − xCuO (NFNB) glass system with 0 ≤ x ≤ 6 mol% have been studied. EPR spectra of all the glass samples exhibit resonance signals characteristic of Cu2+ ions. The values of spin-Hamiltonian parameters indicate that the Cu2+ ions in sodium fluoride-sodium borate (NFNB) glasses were present in octahedral sites with tetragonal distortion. The number of spins (N) participating in resonance was calculated as a function of temperature for NFNB glass sample containing 1 mol% of Cu2+ ions and the activation energy was calculated. From the EPR data, the paramagnetic susceptibility (χ) was calculated at various temperatures and the Curie constant was calculated from the 1/χ − T graph. The optical absorption spectra of these samples show a broad absorption band centered at 13280 cm−1 which is assigned to the 2B1g → 2B2g transition of Cu2+ ions in distorted octahedral sites. The optical band gap energy (Eopt) and Urbach energy (ΔE) are calculated from their ultraviolet edges. It is observed that as the copper ion concentration increases, Eopt decreases while ΔE increases. This has been explained as due to the creation of additional localized states by CuO, which overlap and extend in the mobility gap of the matrix. By correlating the EPR and optical data, the molecular orbital coefficients have been evaluated.