Fast Li-Ion-Conducting Garnet-Related Li 7-3 x Fe x La 3 Zr 2 O 12 with Uncommon I 4̅3 d 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 Ga-stabilized Li ₇ La ₃ Zr ₂ O ₁₂ crystallizes in the acentric cubic space group I 4̅3 d . This structure can be derived by a symmetry reduction of the garnet-type Ia 3̅ d 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 4̅3 d also for Li _{7-3 x} 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 (12 a ) site of space group I 4̅3 d . 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.