Experimental Investigations of the AC-Conductivity in NaTaO3 Ceramic Materials Doped with Cu and Al Metal Ions

Two ceramic samples of sodium tantalate (NaTaO3), doped with metal ions of copper (Cu; sample S1) or aluminum (Al; sample S2), were obtained by the sol-gel method. Complex impedance measurements in the frequency range (200 Hz–2 MHz) and at temperatures between 30 °C and 90 °C allowed identification of a transition temperature from semiconductor-type behavior to conductor-type behavior for each sample (52 °C for sample S1 and 54 °C for sample S2). In the temperature range with semiconductor behavior, the activation energy of each sample was determined. Based on the Mott’s variable-range hopping (VRH) model, the density of localized states at the Fermi level, N(EF), the hopping distance (R) and the hopping energy (W) were determined, for the first time, on NaTaO3 samples doped with Cu or Al metal ions. The increase in N(EF) of sample S2 compared to N(EF) of sample S1 was explained by the decrease in the hopping distance of charge carriers in sample S2 compared to that in sample S1. Additionally, using the correlated barrier hopping (CBH) model, the energy band gap (Wm) and the hopping (crossover) frequency (ωh) at various temperatures were determined. Knowledge of these electrical properties is very important for explaining the electrical conduction mechanisms in metal ion-doped compounds, with perovskite structure being of interest for the use of these materials in the conversion of thermoelectric energy, photocatalytic applications, electronics or other applications.