Structure and lithium ion conductivity of bismuth containing lithium garnets Li5La3Bi2O12 and Li6SrLa2Bi2O12

We report the synthesis, structure and transport properties of the new chemical compositions Li5La3Bi2O12 and Li6SrLa2Bi2O12. Qualitative phase analysis by X-ray powder diffraction patterns in combination with the Rietveld method revealed garnet type compounds as major phases. Whereas Li5La3Bi2O12 was found to be a single-phase material, BiLa2O4.5 could be identified as an impurity phase in the case of Li6SrLa2Bi2O12. Lithium ion conductivities of Li5La3Bi2O12 and Li6SrLa2Bi2O12 were studied by ac impedance method. The grain-boundary contribution to the total (bulk + grain-boundary) resistance is appreciable and amounts to about 54% and 61% for Li5La3Bi2O12 and Li6SrLa2Bi2O12, respectively, at 22 °C. Li6SrLa2Bi2O12 exhibits the highest total (bulk + grain-boundary) and bulk ionic conductivity of 2.0 × 10−5 and 5.2 × 10−5 S/cm, respectively, at 22 °C. The bismuth containing lithium garnet exhibits a maximum cubic lattice constant and the best lithium ion conductivity with low activation energy in the Li5La3M2O12 (M = Ta, Nb, Sb and Bi) series.