Lithium ion conduction in A-site deficient perovskites R1/4Li1/4TaO3 (R=La,Nd, Sm and Y)

New lithium ion conductors of perovskite-type compounds R_1/4Li_1/4TaO₃ (R=La, Nd, Sm and Y) were synthesized, and their crystal structure and lithium ion conductivity were characterized. Symmetry of the perovskite lattice changed from cubic for La_1/4Li_1/4TaO₃ to tetragonal for Nd_1/4Li_1/4TaO₃ and Sm_1/4Li_1/4TaO₃, and to orthorhombic for Y_1/4Li_1/4TaO₃. The alternate ordering of R and Li ions and vacancies at the A-sites was observed for Nd_1/4Li_1/4TaO₃, Sm_1/4Li_1/4TaO₃ and Y_1/4Li_1/4TaO₃. The cube root (V^1/3) of the subcell volume decreased with decreasing the ionic radius of R cations which act as spacers for the perovskite lattice. With decrease in V^1/3 from La_1/4Li_1/4TaO₃ to Y_1/4Li_1/4TaO₃, the bulk conductivity at 400 K decreased from 1.4×10⁻¹ S m⁻¹ to 5.0×10⁻⁷ S m⁻¹ and its activation energy increased from 0.35 eV to 0.85 eV. The subcell volume and the ordering at the A-sites are major factors in influencing lithium ion conduction in these perovskite-type conductors.