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 Li5.5La3Nb1.75In0.25O12 and K-substituted Li5.5La2.75K0.25Nb2O12 were prepared by solid-state reactions at 900, 950, and 1000 degrees 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 Li5La3Nb2O12. 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 angstrom) for smaller Nb5+ (r(CN6): 0.64 angstrom) shows an increase in the lattice parameter from 12.8005(9) to 12.826(1) angstrom at 1000 degrees C. Samples prepared at higher temperatures (950, 1000 degrees C) show mainly bulk lithium ion conductivity in contrast to those synthesized at lower temperatures (900 degrees C). The activation energies for the ionic conductivities are comparable for all samples. Partial substitution of K+ for La3+ and In3+ for Nb5+ in Li5La3Nb2O12 exhibits slightly higher ionic conductivity than that of the parent compound over the investigated temperature regime 25-300 degrees C. Among the compounds investigated, the In-substituted Li5.5La3Nb1.75In0.25O12 exhibits the highest bulk lithium ion conductivity of 1.8 x 10(-4)S/cm at 50 degrees 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) cm(2)/s over the temperature regime 50-200 degrees C, which is extraordinarily high and comparable with liquids. Substitution of Al, Co, and Ni for Nb in Li5La3Nb2O12 was found to be unsuccessful under the investigated conditions. (c) 2005 Elsevier Inc. All rights reserved.