Effect of Zr doping on the semiconducting properties of La0.67 Ca0.33MnO3 ceramics

The transport properties of La_0.67Ca_0.33Mn _1-xZr_xO₃ ceramics exhibit the transition of semiconducting to metallic conductivity at T_P and the magnetic properties observed in the χ-temperature curves show transition from paramagnetic to ferromagnetic at T_C. The coexistence of T_C and T_P are due to the double exchange interaction of two electrons in Mn³⁺-O^2--Mn⁴⁺ and Mn⁴⁺-O^2--Mn³⁺ configuration which brings the system below T_C into a metallic state. Hence it is observed that the Curie temperature T_C is closely related to the sharp decrease in the electrical resistivity of the samples. However, both transition temperatures shift to lower temperatures as zirconium doping increases, indicating the loss of ferromagnetic order and transport properties. As for the transport properties, the semiconductor model ln(σ)~(E_a/kT) was used to explain the conduction mechanism of perovskite manganites above T_P. It was concluded that the total conductivity, σ_tot, consists of the intrinsic and the extrinsic components, such that σ_tot=σ_int+σ_ext. The activation energy increases initially from 0.056 eV to a maximum value of 0.082 eV at x=0.01 and decreases to 0.022 eV at a composition of x=0.20 for the extrinsic region