Mixed ionic–polaronic conduction in copper sodium phosphate glasses containing aluminium oxide

The dielectric properties of sodium phosphate glasses with formula 45P2O5-33ZnO-(20-x)Na2O-xCuO-2Al2O3; x = 0, 1, 2, 3, 4 and 5, have been investigated over a frequency range of 1.05–100 kHz in a vacuum medium (10−5 torr). Some dielectric parameters, such as dielectric constant ϵ′, dielectric loss ϵ″, dielectric loss tangent (tanα) and AC conductivity, have been estimated. High dependency of the dielectric constant on the CuO concentration is observed in the investigated phosphate glasses. The dielectric constant ϵ′, dielectric loss and AC conductivity exhibit an initial decrease with increasing CuO content, followed by a strong rise at 3 mol% of CuO. The AC conductivity measurements show that these phosphate glasses are mixed ionic–polaronic conductors. By the aid of transmission spectra of these glasses, the absorption coefficient and then the optical direct bandgap are estimated. The optical bandgap is found to decrease with increasing CuO content, indicating the formation of non-bridging oxygens.