Correlation between electrical properties and thermodynamic stability of ACoO(3-delta) perovskites (A = La, Pr, Nd, Sm, Gd)

Article

关键词: SPIN-STATE TRANSITION;    INSULATOR-METAL TRANSITIONS;    ELECTRONIC-STRUCTURE;    NEUTRON-DIFFRACTION;    MAGNETIC-PROPERTIES;    RCOO3 R;    TEMPERATURE;    LACOO3;    PHASE;    NONSTOICHIOMETRY;   

DOI  :  10.1103/PhysRevB.84.085113

来源: SCIE

【 摘 要 】

For perovskites with the general formula ACoO(3-delta) (A = La, Pr, Nd, Sm, and Gd) the influence of the A-site cation on the electrical conductivity, electronic structure, thermodynamic stability, and oxygen stoichiometry was studied. The perovskite oxide powders were produced by a combined citric acid and ethylenediaminetetraacetic acid complexing method. Ceramic specimens sintered at 1100 degrees C in air were single-phase perovskites. With increasing temperature, the electrical conductivity shows three discrete regimes. All compositions show semiconductivity up to a transition temperature of similar to 300 degrees C-450 degrees C and then behave like metallic conductors. The activation energies for the semiconductivity, as well as the transition temperatures to the metallic-like conduction, decrease monotonically with increasing pseudocubic lattice parameters, i.e., with increasing ionic radii of the A cation. This behavior correlates with decreasing oxygen nonstoichiometry and increased thermodynamic stability. The highest conductivity and the lowest activation energy of 0.66 eV were found for LaCoO3-delta, which also had the lowest semiconductor-metal transition temperature at 269 degrees C, the lowest oxygen nonstoichiometry of delta = 0.008, and the highest Gibbs free energy change for the decomposition reaction of 42.37 kJ/mol at 850 degrees C. GdCoO3-delta had the highest oxygen nonstoichiometry with delta = 0.032, a high activation energy of 1.19 eV for the semiconductivity with a high transition temperature at 452 degrees C, and the lowest Gibbs free energy change of 26.54 kJ/mol at 850 degrees C. X-ray absorption spectroscopy data imply an increasing Co low-spin character with decreasing cation radius from La to Gd, while an increase in temperature increases the number of holes or Co 3d bandwidth. This correlates well with the electrical conductivity data.

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