Electrical characterization of Co-doped La2CuO4+δ at elevated temperature

A series of samples of the La2Cu1−xCoxO4+δ solid solution with the composition x = 0.00, 0.025, 0.05, 0.10, and 0.20 were prepared by polymeric precursor synthesis. The influence of increasing Co content on the crystal structure was investigated using the Rietveld profile method. The electrical conductivity and thermoelectric power measurements were performed in the temperature range of 300–1173 K. The transition of orthorhombic-to-tetragonal structure and the change of lattice parameters with Co doping were interpreted from the relief of the Jahn-Teller distortion of the CuO6 octahedra having a d9 electronic configuration. The electrical conductivity of the corresponding compounds decreased with an increase of Co doping at low temperature, while thermoelectric power increased. However, at elevated temperature, the electrical conduction showed an inverse trend, exhibiting increased values in electrical conductivity and decreased values in thermoelectric power, with increasing Co content. These behaviors in electrical conduction with Co doping in the La2CuO4+δ were attributed to the ability of Co ions to localize charge carriers derived by an excess oxygen and also to an effect of hole doping in the σ∗x2−y2 conduction band by fewer 3d electrons of Co ions.