Effect of B-site substitution on the crystal structure, electrical conductivity and oxygen transport properties of La_0.5Sr_0.5M_0.2Fe_0.8O_3-δ (M = Co, Al, and Zn) perovskite

The crystal structure, electrical conductivity, and oxygen transport coefficients, namely chemical bulk diffusion coefficient (D_chem), and surface exchange coefficient (K_chem) of La_0.5Sr_0.5M_0.2Fe_0.8O_3-δ (M ​= ​Co, Al, Zn) perovskite oxides systematically investigated. B-site substituted oxides exhibited a distorted rhombohedral crystal structure. The lattice volume and oxygen nonstoichiometry in La_0.5Sr_0.5Al_0.2Fe_0.8O_3-δ (LSAF) or La_0.5Sr_0.5Zn_0.2Fe_0.8O_3-δ (LSZF) surrogates increases as compared to La_0.5Sr_0.5Co_0.2Fe_0.8O_3-δ (LSCF). X-ray photoelectron spectroscopic (XPS) study indicates the variation of B-site cations oxidation state and oxygen vacancy concentration of the synthesized perovskite oxides as a function of substituents. The microstructural study on samples sintered under identical conditions revealed grain size refinement in the LSAF sample as compared to others. Studies on electrical conductivity and oxygen transport properties of LSAF samples showed an increase in conductivity and a decrease in D_chem and K_chem parameters with increasing the grain size. LSAF samples exhibited enhanced electrical conductivity and D_chem and K_chem values as compared to LSZF samples when the grain size kept identical and correlated with the defects present in the samples. The study further suggests that variation in the functional properties of these perovskite oxides when sintered under identical conditions is a combined effect of grain size and composition.