Structural, magnetic, and transport properties of the electron-doped perovskites cobalt oxides La_1-xTe_xCoO₃ (0≤x≤0.25)

The structural, magnetic, and transport properties of the electron-doped perovskites cobalt oxides La_1-xTe_xCoO₃ (0≤x≤0.25) have been investigated. For the structural properties, the room temperature structural transition from rhombohedra (R3C) to orthorhombic (Pbnm) symmetry is found in these samples with x≥0.10 by the Rietveld refinement of x-ray powder diffraction patterns, which is ascribed to the smaller ionic radius of Te. As for the magnetic properties, firstly the spin state transition of Co³⁺ ions is observed in all samples, and the transition temperature shifts up to higher temperature with Te doping, which is ascribed to the smaller ionic radius and the strong acidity of Te. Secondly, it is interesting that there exists paramagnetic to ferromagnetic phase transition in the Te-doped samples. It is suggested that such a phase transition results from the introduction of Co²⁺ ions and the appearance of double exchange interaction between Co²⁺ and Co³⁺ due to Te doping. Moreover, Curie temperature T_C increases with the increase of the Te doping level x (for x≤0.10), and then decreases with the further increase of x. About the transport properties, the temperature dependence of the resistivity ρ shows that all samples display semiconducting behavior in the whole measured temperature region. And it can be seen that the resistivity first decreases with increasing Te doping level from x=0 to 0.10, then increases as x≥0.15. Such variations of T_C and ρ with x are suggested to be related to the structure transition.