  We report on a novel approach to synthesize cubic-phase fast ionic conducting
garnet-type solid state electrolytes based on Bi doped Li7La3Zr2O12 (LLZO). Bi
aliovalent substitution into LLZO utilizing the Pechini processing method is
successfully employed to synthesize Li7-xLa3Zr2-xBixO12 compounds. Ionic
conductivities up to 2.0 x 10-4 S/cm are achieved in structures not fully
densified. Cubic phase Li6La3ZrBiO12 powders are generated in the temperature
range from 650 {\deg}C to 900 {\deg}C in air. In contrast, in the absence of Bi
and under identical synthesis conditions, the cubic garnet phase of
Li7La3Zr2O12 is not formed below 700 {\deg}C while a transformation to the
tetragonal phase is observed at 900 {\deg}C for the un-doped compound. The
critical role of Bi in lowering the formation temperature of the garnet cubic
phase and the improvements in ionic conductivity is investigated in this work
through microstructural studies and AC impedance measurements. We ascribe the
effect of Bi doping in achieving these remarkable improvements to significant
enhancements at lower temperatures in the kinetics of the solid-state reaction
resulting in explosive grain growth and densification of the garnet. Moreover,
XAS is utilized to identify the specific atomic site where Bi is incorporated
in the LLZO garnet crystalline structure.
