Electrical conductivity and oxygen nonstoichiometry in the double B mixed La0.6Ca0.4Mn1-xCoxO3-δ perovskite system

Double B mixed perovskite-type series in the La0.6Ca0.4Mn1-xCoxO3-delta system with x=0-1.0 was synthesized in air by solid state reactions at 1100 degrees C from La2O3, MnO2, CoO simple oxides and CaCO3. Replacement of Mn cations by Co is characterized by U-shaped dependence of the conductivity versus Co content with a small maximum at x=0.3. Lower conductivities of the Co-Mn containing compositions compared to La0.6Ca0.4MnO3-delta and La0.6Ca0.4CoO3-delta are explained by stronger polarization of the double B center dot center dot center dot -Mn(alpha+gamma)+-O alpha--Co(alpha-gamma)+-O alpha-- center dot center dot center dot fragments of the center dot center dot center dot -O alpha--Mn alpha+-O alpha--Mn alpha+-O alpha--Mn(alpha+gamma)+-O alpha--Co(alpha-gamma)+-O alpha-- center dot center dot center dot chains in comparison with the single B center dot center dot center dot -O alpha--Mn alpha+-O alpha--Mn alpha+-O alpha-- center dot center dot center dot and center dot center dot center dot -O alpha--Co alpha+-O alpha(-)-Co alpha+-O alpha- center dot center dot center dot fragments, because of different electronegativity values of Co and Mn. The double exchange theory gives strong explanation for this phenomenon. Type and level of the electrical conductivity of the La0.6Ca0.4Mn1-xCoxO3-delta series are functions of the [Mn4+]/[Mn3+] and [Co3+]/[Co2+] ratios. Mn4+ and Co2+ cations are possible point defects which determinate the observed p- or n-type conductivity of the compositions, respectively. (C) 2009 Elsevier B.V. All rights reserved.