Topochemical Synthesis and Formation Mechanism of Garnet Multimetal Fluorides

The synthesis of multimetal fluorides (MMFs) is challenged by the usage of toxic fluoride precursors and their thermodynamic instability at higher temperatures. Here, we demonstrate a new topochemical reaction pathway to tailor garnet-type single- and multimetal fluorides from double perovskite (DP) hosts. Our combined X-ray diffraction, Fourier-transformed infrared, and nuclear magnetic spectroscopic techniques reveal the transformation pathway as DP-(NH4)3(M/M′)3+F6 to DP-(NH4)2(Na/Li)(M/M′)3+F6 to garnet-Na3Li3(M/M′)2F12 ((M/M′)3+ = Al3+, Fe3+, Cr3+ and V3+) through ion-exchange reaction between NH4+ and Li+/Na+ ions. The garnet MMF-Na3Li3(Fe0.33Cr0.33V0.33)2F12 catalyst displays ultralow overpotential (η500 of 245 mV) with higher durability.