A and B-site substitution of the solid electrolyte LaGaO3 and LaAlO3 with the alkaline-earth oxides MgO and SrO

Solid electrolytes LaGaO3 and LaAlO3 with various amounts of SrO and MgO addition were synthesized using the solid-state reaction method. A significant difference in the substitution of MgO and SrO on the B- and A-site, respectively, of perovskite lattice was observed between the LaGaO3 and LaAlO3 systems. In the LaGaO3 system, the solubility of Mg ions in the Ga-cation sublattice was found to be 20%. However, only less than 10% of the La-cation sublattice could be substituted by Sr ions. With the co-substitution of SrO and MgO, the solubility of Sr ions was significantly enhanced by the addition of Mg ions. The solubility enhancement of Sr ions is attributed to the expanded lattice caused by the addition of Mg ions. In the LaAlO3 system, the solubility of Mg ions on Al-cation sites was less than 10%. The reason for this low solubility of MgO in LaAlO3 is simply due to the 35% mismatch of ionic radius between the Mg (r(Mg) = 0.72 Angstrom) and Al (r(Al) = 0.535 Angstrom) cations. The solubility of Sr ion in the La-cation sublattice was as high as 20%. With co-substitution of SrO and MgO in LaAlO3, the enhancement of MgO solubility was also observed. However, further addition of MgO lowered the solubility of SrO from 20 to 10%. This result can be rationalized by the reduced distance between the Mg and Sr ions that may cause an unfavorable cation-cation repulsion in the perovskite structure. (C) 2003 Elsevier B.V. All rights reserved.