It was noted that the dissolution of divalent cations played an important role in the enhancement of conductivity of perovskite-based LaAlO3 and LaGaO3 oxygen conductors. In LaAlO3, the solubility of MgO was less than 10%; due to the mismatch of the ionic radii between Mg and Al cations. The substitution of Sr ions in the La-cation sub-lattice was as high as 20%. With double-doping of SrO and MgO in LaAlO3, an enhancement of MgO solubility was also observed. Further additions of MgO tended to lower the solubility of the Sr ion from 20 to 10%. This could be rationalized in terms of a reduced distance, between Mg ions and Sr ions, that caused cation–cation repulsion in the perovskite structure. In singly-doped LaGaO3, the solubility of MgO was 20%. However, less than 10% of the La-cation sub-lattice could be replaced by Sr ions. With double-doping of SrO and MgO, the solubility of SrO was significantly enhanced by the addition of MgO. It was suggested that the solubility enhancement of SrO was due to the expanded lattice caused by the addition of MgO. Within the solubility limit of the aliovalent cations, the ionic conductivities of LaAlO3 and LaGaO3 increased with increasing concentration of foreign cations. When the solubility limits in doped LaAlO3 and LaGaO3 were reached, segregation of the second phase tended to lower sharply the ionic conductivity. The activation energy for ionic conduction also depended upon the ionic radii of foreign cations, by affecting the space available for the transport of O ions.

Comparison of Dissolution Behavior and Ionic Conduction between Sr and/or Mg Doped LaGaO3 and LaAlO3. T.Y.Chen, K.Z.Fung: Journal of Power Sources, 2004, 132[1-2], 1-10