Fast Li-Ion-Conducting Garnet-Related Li7-3xFexLa3Zr2O12 with Uncommon I(4)over-bar3d Structure

Fast Li-ion-conducting Li oxide garnets receive a great deal of attention as they are suitable candidates for solid-state Li electrolytes. It was recently shown that Gastabilized Li7La3Zr2O12 crystallizes in the acentric cubic space group I (4) over bar 3d. This structure can be derived by a symmetry reduction of the garnet-type Ia (3) over bard structure, which is the most commonly found space group of Li oxide garnets and garnets in general. In this study, single-crystal X-ray diffraction confirms the presence of space group I (4) over bar 3d also for Li7-3xFexLa3Zr2O12. The crystal structure was characterized by X-ray powder diffraction, single-crystal X-ray diffraction, neutron powder diffraction, and MoBbauer spectroscopy. The crystal chemical behavior of Fe3+ in Li7La3Zr2O12 is very similar to that of Ga3+. The symmetry reduction seems to be initiated by the ordering of Fe3+ onto the tetrahedral Lil (12a) site of space group I (4) over bar 3d. Electrochemical impedance spectroscopy measurements showed a Li-ion bulk conductivity of up to 1.38 X 10(-3) S cm(-1) at room temperature, which is among the highest values reported for this group of materials.