Mixed alkali effect in borate glasses - electron paramagnetic resonance and optical absorption studies in Cu2 dopedxNa2O– (30 x)K2O– 70B2O3glasses

The mixed alkali borate glasses xNa₂O-(30 - x)K₂O-70B₂O₃ (5 \≤ x \≤ 25, doped with 0.5 mol% of CuO, have been investigated, using electron paramagnetic resonance (EPR) and optical absorption techniques, as a function of mixed alkali content, to look for the 'mixed alkali effect' (MAE) on the spectral properties of the glasses. The EPR spectra of all the investigated samples exhibit resonance signals which are characteristic of the Cu²⁺ ions in octahedral sites with tetragonal distortion. From the observed EPR spectra, the spin-Hamiltonian parameters have been determined. It is observed that the spin-Hamiltonian parameter g\\| goes through a minimum around x = 10-15 whereas A\\| goes through a maximum around x = 15 showing the MAE. The number of spins participating in resonance (N₂) and the calculated paramagnetic susceptibilities (\χ) exhibit a shallow minimum around x = 20 showing the MAE in these glasses. The optical absorption spectrum of the x = 5 glass exhibits two bands: a strong band centred at 14 240 cm⁻¹ corresponding to the transition (²B₁g \→ ²B₂g) and a weak band on the higher energy side at 22 115 cm⁻¹ corresponding to the transition (²B₁g \→ ²Eg). With x > 5, the higher energy band disappears and the lower energy band shifts slightly to the lower energy side. By correlating the EPR and optical absorption data, the molecular orbital coefficients \α² and \β²₁ are evaluated for the different glasses investigated. The values indicate that the in-plane \σ bonding is moderately covalent while the in-plane \π bonding is significantly ionic in nature; these exhibit a minimum with x = 15, showing the MAE. The theoretical values of optical basicity of the glasses have also been evaluated. From optical absorption edges, the optical bandgap energies have been calculated and are found to lie in the range 3.00-3.40 eV. The physical properties of the glasses studied have also been evaluated with respect to the composition.