Effect of Li addition on crystal structure and phase separation of highly defective (La,Li)TaO3 solid electrolytes

La1/3TaO3 was found to exhibit a perovskite structure in which two-thirds of A-cation sites are vacant. An ordered arrangement of vacancies causes the formation of a superstructure with doubled c-axis compared with a perovskite subcell. With small amount of Li addition, a superstructure with vacancies located at the (002) plane was still present. When Li ions were added into more than 20% of A-cation sites, the superstructure disappeared and the perovskite structure also changed from tetragonal to near cubic. When more than 60% of A-cation sites were incorporated by Li ions, a phase separation of cubic La0.166Li0.5TaO3 perovskite and rhombohedral LiTaO3 was observed. The phase separation was caused by the big difference of ionic radius between La and Li ions. The microstructure and microchemical analysis of (La,Li)TaO3 were examined using scanning electron microscopy and EDX.