Effect of Zr doping on the semiconducting properties of La/sub 0.67/Ca/sub 0.33/MnO/sub 3/ ceramics

The transport properties of La/sub 0.67/Ca/sub 0.33/Mn/sub 1-x/Zr/sub x/O/sub 3/ ceramics exhibit the transition of semiconducting to metallic conductivity at T/sub P/ and the magnetic properties observed in the /spl chi/-temperature curves show transition from paramagnetic to ferromagnetic at T/sub C/. The coexistence of T/sub C/ and T/sub P/ are due to the double exchange interaction of two electrons in Mn/sup 3+/-O/sup 2-/-Mn/sup 4+/ and Mn/sup 4+/-O/sup 2-/-Mn/sup 3+/ configuration which brings the system below T/sub C/ into a metallic state. Hence it is observed that the Curie temperature T/sub 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(/spl sigma/)/spl sim/(E/sub a//kT) was used to explain the conduction mechanism of perovskite manganites above T/sub P/. It was concluded that the total conductivity, /spl sigma//sub tot/, consists of the intrinsic and the extrinsic components, such that /spl sigma//sub tot/=/spl sigma//sub int/+/spl sigma//sub 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.