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

Abstract

GdCoO_3-δ, Gd_0.975Na_0.025CoO_3-δ, Gd_0.98K_0.02CoO_3-δ, Gd_0.98Ca_0.02CoO_3-δ, and GdCo_0.99Mg_0.01O_3-δ 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 GdCoO_3-δ ceramic (a = 5.223 Å, b = 5.389 Å and c = 7.451 Å), 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 μm) 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 GdCoO_3-δ 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, Gd_0.98Ca_0.02CoO_3-δ had the largest electrical conductivity at all measured temperatures among the compositions studied, which was 144% higher at 500 °C and 16% higher at 800 °C compared to those of GdCoO_3-δ ceramic (500 °C: 133.3 S·cm⁻¹; 800 °C: 692.4 S·cm⁻¹). The substantial increase in the electrical conductivity of the doped GdCoO_3-δ ceramics is due to the electronic compensation of acceptor doping, ${\mathrm{N}a}_{Gd}^{"}$, ${K}_{Gd}^{"}$, and ${Ca}_{Gd}^{\prime }$, which resulted in the formation of hole carriers and the elimination of oxygen vacancies (${V}_o^{\bullet \bullet }$), which generated additional Co⁴⁺ (${Co}_{Co}^{\bullet }$).