Fast Li-Ion-Conducting Garnet-Related Li(7–3x)Fe(x)La(3)Zr(2)O(12) with Uncommon I4̅3d Structure

[Image: see text] 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 Ga-stabilized Li(7)La(3)Zr(2)O(12) crystallizes in the acentric cubic space group I4̅3d. This structure can be derived by a symmetry reduction of the garnet-type Ia3̅d 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 I4̅3d also for Li(7–3x)Fe(x)La(3)Zr(2)O(12). The crystal structure was characterized by X-ray powder diffraction, single-crystal X-ray diffraction, neutron powder diffraction, and Mößbauer spectroscopy. The crystal–chemical behavior of Fe(3+) in Li(7)La(3)Zr(2)O(12) is very similar to that of Ga(3+). The symmetry reduction seems to be initiated by the ordering of Fe(3+) onto the tetrahedral Li1 (12a) site of space group I4̅3d. Electrochemical impedance spectroscopy measurements showed a Li-ion bulk conductivity of up to 1.38 × 10(–3) S cm(–1) at room temperature, which is among the highest values reported for this group of materials.