Synthesis and Electrical Properties of K- and Pr-Substituted LaGaO₃ and LaInO₃ Perovskites

Replacement of divalent Sr²⁺ ions by monovalent K⁺ ions in La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSGM) decreases the electrical conductivity of LSGM and increases the activation energy to 1.42 eV. Substitution of La by substantial amounts of up to 50 atom % of Pr in LSGM yields new mixed oxide ion and electronic conductors. Substitution of 50 atom % Pr for La in the In-analog of LSGM, La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSIM), does not increase the electrical conductivity. Among the investigated oxides, La_0.4Pr_0.4Sr_0.2In_0.8Mg _0.2O_2.8 exhibits the lowest activation energy of 0.44 eV and the highest electrical conductivity of 3.17 × 10^-5 S/cm at 200°C. Oxygen partial pressure dependence of the electrical conductivity reveals that LSIM and Pr-substituted LSIM perovskites are mixed oxide ion and p-type electronic conductors at high oxygen partial pressures. The electronic conductivity increases with increasing Pr content in LSIM, Open-circuit voltage measurements employing La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 as separator in the galvanic cell H₂||air show that the average transference number (t₀^2-) for oxide ion conduction is 0.78-0.90 in the temperature range 450-650°C. Powder X-ray diffraction data reveal that LSIM and Pr-substituted LSIM perovskites are not stable at low oxygen partial pressures of about 10^-22 atm, while Pr-substituted LSGM retains the cubic perovskite structure. Accordingly, the new materials reported here may find application as electrode materials for solid oxide fuel cells and oxygen sensors.