First Mixed-Metal Fluoride Pyrochlores Obtained by Topotactic Oxidation of Ammonium Fluorides under F₂ Gas

Metal fluorides with 3D open structures, pyrochlore (pyr) or hexagonal tungsten bronze (HTB), are promising materials as positive electrodes for rechargeable batteries or catalysts. Herein, we have developed a two-step synthesis procedure to obtain new anhydrous mixed-metal-cation fluorides crystallizing in the pyrochlore structure. The first step consists of preparing mixed-metal ammonium fluorides (NH₄)­M²⁺Fe³⁺F₆ (M = Mn, Fe, Co Ni) using different synthesis strategies. For M = Mn, three allotropic varieties of (NH₄)­Mn²⁺Fe³⁺F₆ are obtained; two phases adopt the expected pyrochlore network with either the cubic Fm3̅m or the orthorhombic Pnma space group, and the third phase exhibits a 3D network with narrow pseudotriangular cavities. ⁵⁷Fe Mössbauer spectrometry indicates that the crystal structures are governed by the Fe³⁺/Mn²⁺ cationic order or disorder. The second step is a topotactic oxidation of pyr-(NH₄)­M²⁺Fe³⁺F₆ under a molecular F₂ flow. To better understand the reaction mechanism, the topotactic oxidation was followed by thermogravimetry, XRD, FTIR, and Mössbauer spectrometry. The successful synthesis of the first anhydrous pyr-M³⁺_0.5Fe_0.5F₃ provides a new route to prepare anhydrous mixed-metal fluorides pyrochlore with empty cavities of the open framework.