Effect of sintering on the ionic conductivity of garnet-related structure Li₅La₃Nb₂O₁₂ and In- and K-doped Li₅La₃Nb₂O₁₂

Garnet-structure related metal oxides with the nominal chemical composition of Li₅La₃Nb₂O₁₂, In-substituted Li_5.5La₃Nb_1.75In_0.25O₁₂ and K-substituted Li_5.5La_2.75K_0.25Nb ₂O₁₂ were prepared by solid-state reactions at 900, 950, and 1000 °C using appropriate amounts of corresponding metal oxides, nitrates and carbonates. The powder XRD data reveal that the In- and K-doped compounds are isostructural with the parent compound Li₅La ₃Nb₂O₁₂. The variation in the cubic lattice parameter was found to change with the size of the dopant ions, for example, substitution of larger In³⁺(r_CN6: 0.79 Å) for smaller Nb⁵⁺ (r_CN6: 0.64 Å) shows an increase in the lattice parameter from 12.8005(9) to 12.826(1) Å at 1000 °C. Samples prepared at higher temperatures (950, 1000 °C) show mainly bulk lithium ion conductivity in contrast to those synthesized at lower temperatures (900 °C). The activation energies for the ionic conductivities are comparable for all samples. Partial substitution of K⁺ for La³⁺ and In³⁺ for Nb⁵⁺ in Li₅La₃Nb ₂O₁₂ exhibits slightly higher ionic conductivity than that of the parent compound over the investigated temperature regime 25-300 °C. Among the compounds investigated, the In-substituted Li_5.5La ₃Nb_1.75In_0.25O₁₂ exhibits the highest bulk lithium ion conductivity of 1.8×10^-4 S/cm at 50 °C with an activation energy of 0.51 eV. The diffusivity ("component diffusion coefficient") obtained from the AC conductivity and powder XRD data falls in the range 10^-10-10^-7 cm²/s over the temperature regime 50-200 °C, which is extraordinarily high and comparable with liquids. Substitution of Al, Co, and Ni for Nb in Li ₅La₃Nb₂O₁₂ was found to be unsuccessful under the investigated conditions.