Fast Li-Ion-Conducting Garnet-Related Li_{7–3<i>x</i>}Fe_<i>x</i>La₃Zr₂O₁₂ with Uncommon <i>I</i>4̅3<i>d</i> 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 Li₇La₃Zr₂O₁₂ crystallizes in the acentric cubic space group <i>I</i>4̅3<i>d</i>. This structure can be derived by a symmetry reduction of the garnet-type <i>Ia</i>3̅<i>d</i> 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>I</i>4̅3<i>d</i> also for Li_{7–3<i>x</i>}Fe_<i>x</i>La₃Zr₂O₁₂. 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 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 Li1 (12<i>a</i>) site of space group <i>I</i>4̅3<i>d</i>. 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