Effects of Ca2+, Mg2+, Na+, and K+ substitutions on the microstructure and electrical properties of GdCoO3 ceramics

GdCoO3-delta, Gd0.975Na0.025CoO3-delta, Gd0.98K0.02CoO3-delta, Gd0.98Ca0.02CoO3-delta, and GdCo0.99Mg0.01O3-delta ceramics were prepared via a solid-state reaction route. Among the dopants studied, substitution with Ca2+ slightly enhanced the densfication of GdCoO3 ceramics. All the lattice parameters of the doped ceramics were larger than those of pure GdCoO3-delta ceramic (a = 5.223 angstrom, b = 5.389 angstrom and c = 7.451 angstrom), and their cell volumes increased by 0.30% to 1.40% because the substitution ions were larger in size. X-ray diffraction and scanning electron microscopy results indicate that no second phase is present. The average grain size of the GdCoO3 ceramics (7.6 mu m) slightly increased by the Na+, K+ and Mg2+ substitutions and decreased by the Ca2+ substitution. In all cases, the intergranular fracture surfaces revealed the presence of trapped pores due to rapid grain growth. The oxidation states and percentages of Co ions were determined from the Co 2p X-ray photoelectron spectra. Na+, K+, Ca2+, and Mg2+ substitution in the GdCoO3-delta ceramic resulted in slight oxidation of the Co ions accompanied by a decrease in oxygen vacancies. After porosity correction using the Bruggeman effective medium approximation, Gd0.98Ca0.02CoO3-delta had the largest electrical conductivity at all measured temperatures among the compositions studied, which was 144% higher at 500 degrees C and 16% higher at 800 degrees C compared to those of GdCoO3-delta ceramic (500 degrees C: 133.3 S.cm(-1); 800 degrees C: 692.4 S.cm(-1)).The substantial increase in the electrical conductivity of the doped GdCoO3-delta ceramics is due to the electronic compensation of acceptor doping, Na-Gd(}), K-Gd(}), and Ca-Gd', which resulted in the formation of hole carriers and the elimination of oxygen vacancies (V-o(center dot center dot)), which generated additional Co4+ (Co-Co).