This study reports on oxygen nonstoichiometry, electronic conductivity and lattice expansion of three compositions as function of T and PO2 in the (La1−xSrx)sCo1−yNiyO3−δ (x=0.1, y=0.4; x=0.1, y=0.3; x=0.2, y=0.2) materials system. The nonstoichiometry data were successfully fitted using the itinerant electron model which indicates the existence of delocalized electronic states. This was also reflected in the high electronic conductivities, above 1000 S cm−1, measured for all three compositions. The electronic conductivity was shown to decrease linearly with the oxygen nonstoichiometry parameter, δ, supporting that the conductivity is dependent on p-type charge carriers. Comparing calculated p-type mobilities with data reported in literature on La1−xSrxCoO3 indicated that Ni-substitution into (La1−xSrx)sCoO3−δ increases the p-type mobility. The electronic conductivity was also found to be dependent on intrinsic charge related to spin excitations and Ni substitution rather than the p-type charge. A conductivity mechanism is hypothesized including a metallic like conductivity of the p-type charge and a small polaron conductivity of the intrinsic charge. Lattice expansion as function of T and δ was successfully described using first and second order thermal and chemical expansion coefficients. Substituting 10% Co with Ni in (La0.6Sr0.4)0.99CoO3−δ was found to decrease the apparent thermal expansion with about 25%.
Electronic conductivity as function of temperature for (La0.8Sr0.2)0.99Co0.8Ni0.2O3−δ. A model with metallic like conductivity for the extrinsic p-type charge carrier (due to Sr substitution) and small polaron conductivity for intrinsic charge carriers (due to Co spin transitions) was found to best describe the total conductivity.
Keywords: Electronic conductivity; Lattice expansion; Mobility; Itinerant electron model; Lanthanum strontium cobaltite; Lanthanum strontium cobalt nickel oxide