Electrical transport and dielectric relaxation in 5Fe2O3–40ZnO-55P2O5 iron zinc phosphate bulk glass

Electrical Impedance Spectroscopy (EIS) and Optical Spectroscopy (OS) experimental techniques have been used to investigate some basic and important aspects of the electronic band structure, electrical transport and dielectric relaxation in 5Fe₂O₃–40ZnO–55P₂O₅ iron zinc phosphate bulk glass. The temperature dependence of the dc electrical conductivity, as determined by EIS, showed a simple Arhenius behaviour, with activation energy of ∼1 eV, a value shared by a large number of other glassy systems. The EIS measurements revealed also a number of discrete dielectric relaxation processes, some of them possessing a non-exponential “universal” behaviour. The temperature dependence of the characteristic frequency of one of these dielectric relaxation processes was found to be related to the observed values of dc electrical conductivity at the same temperatures through Barton-Namikawa-Nakajima (BNN) relation. Based on the analysis of both the optical and the electrical measurements, it is concluded that the dc electrical transport is due to hole small polaron hopping, but the Fe sites are not the hopping sites. Rather, the added Fe atoms in their possible different charge states within the glassy matrix act as strongly localised acceptor sites (bands) and the dc electrical transport then takes place via self-trapped holes, originating from the remaining singly occupied electron states (holes) at the top of the valence band tails in studied glass. The existence of “defect” acceptor bands in the vicinity of the valence bands in wider band gap glassy materials and subsequent hole self-trapping is proposed to be a general trend and the cause of largely p-type behaviour and polaronic type of electrical conduction in these systems.