Topochemical\nSynthesis and Formation Mechanism of\nGarnet Multimetal Fluorides

The synthesis of multimetal fluorides\n(MMFs) is challenged\nby the\nusage of toxic fluoride precursors and their thermodynamic instability\nat higher temperatures. Here, we demonstrate a new topochemical reaction\npathway to tailor garnet-type single- and multimetal fluorides from\ndouble perovskite (DP) hosts. Our combined X-ray diffraction, Fourier-transformed\ninfrared, and nuclear magnetic spectroscopic techniques reveal the\ntransformation pathway as DP-(NH<sub>4</sub>)<sub>3</sub>(M/M′)<sup>3+</sup>F<sub>6</sub> to DP-(NH<sub>4</sub>)<sub>2</sub>(Na/Li)(M/M′)<sup>3+</sup>F<sub>6</sub> to garnet-Na<sub>3</sub>Li<sub>3</sub>(M/M′)<sub>2</sub>F<sub>12</sub> ((M/M′)<sup>3+</sup> = Al<sup>3+</sup>, Fe<sup>3+</sup>, Cr<sup>3+</sup> and V<sup>3+</sup>) through ion-exchange\nreaction between NH<sub>4</sub><sup>+</sup> and Li<sup>+</sup>/Na<sup>+</sup> ions. The garnet MMF-Na<sub>3</sub>Li<sub>3</sub>(Fe<sub>0.33</sub>Cr<sub>0.33</sub>V<sub>0.33</sub>)<sub>2</sub>F<sub>12</sub> catalyst displays ultralow overpotential (η<sub>500</sub> of\n245 mV) with higher durability.