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

Abstract

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+ 0.5Fe0.5F3 provides a new route to prepare anhydrous mixed-metal fluorides pyrochlore with empty cavities of the open framework.

Chloride Reduction of Mn3+ in Mild Hydrothermal Synthesis of a Charge Ordered Defect Pyrochlore, CsMn2+Mn3+F6, a Canted Antiferromagnet with a Hard Ferromagnetic Component

“…For that reason, the disordered cubic phase can be obtained under either high temperature conditions in flux reactions with T > 1000 °C, where the effective size of the A cations is high due to disproportionally high increase in their thermal motion in the resulting crystal structure, or by entangling the mobility of the M cations (Figure ). A good illustration of the latter is the recently obtained (NH 4 )­MnFeF 6 defected pyrochlore, which can be synthesized as a cubic phase via ball milling of the fluoride precursors . Another example is the decomposition of [Co 3+ (NH 3 ) 6 ]­AlF 6 at 280 °C, which results in the formation of cubic NH 4 Co 2+ AlF 6 , whereas a reaction between CoF 2 , AlF 3 , and NH 4 F under hydrothermal conditions at 400 °C yields an ordered orthorhombic phase .…”

“…fluoride precursors. 54 Another example is the decomposition of [Co 3+ (NH 3 ) 6 ]AlF 6 at 280 °C, which results in the formation of cubic NH 4 Co 2+ AlF 6 , whereas a reaction between CoF 2 , AlF 3 , and NH 4 F under hydrothermal conditions at 400 °C yields an ordered orthorhombic phase. 39 Moreover, the cubic phase of NH 4 CoAlF 6 can be transformed to the orthorhombic one by annealing it at 350 °C for 1 week, which enables the mobility of the M cations in the solid state, illustrating the metastable nature of the cubic phase.…”

Fluorinated Materials as Positive Electrodes for Li- and Na-Ion Batteries

“…Finally, a cationic substitution brings a priori a gain in structural stability during lithium insertion with a capacity of 150 mAh g –1 after 200 cycles for Fe 0.9 Co 0.1 F 3 (4.5–1.8 V, 0.1C, LIB), and for Fe 1.95 Cr 0.05 F 5 ·H 2 O, of probable formulation Fe 0.975 Cr 0.025 F 3 ·0.5H 2 O, a capacity of 171 mAh g –1 after 100 cycles (4.0–1.0 V, 0.1C, NIB). , Following this idea, the first mixed metal fluorides pyr-M 3+ 0.5 Fe 3+ 0.5 F 3 were stabilized with a two-step synthesis: new NH 4 M 2+ Fe 3+ F 6 ammonium fluorides were synthesized through several techniques and a topotactic oxidation under F 2 flow was performed to obtain pyrochlore promising cathode materials …”

“…266,280 Following this idea, the first mixed metal fluorides pyr-M 3+ 0.5 Fe 3+ 0.5 F 3 were stabilized with a two-step synthesis: new NH 4 M 2+ Fe 3+ F 6 ammonium fluorides were synthesized through several techniques and a topotactic oxidation under F 2 flow was performed to obtain pyrochlore promising cathode materials. 281 2.4.7. K 0.6 FeF 3 Tetragonal Tungsten Bronze Type.…”

A Geometrically Frustrated Family of MIIMIIIF5(H2O)2 Mixed–Metal Fluorides with Complex Magnetic Interactions

“…An additional outcome of this stacking is the creation of a kagome-type network for the A cations (Figure ). This kagome arrangement is the origin of the observed magnetic frustration in these materials, which is of interest due to the unusual magnetic ground states and physical properties that are related to it. − Magnetic frustration occurs primarily from competing antiferromagnetic spin interactions caused by the topology of the lattice that hinders a simultaneous antiferromagnetic arrangement of coupled magnetic moments, and the types of structures that exhibit magnetic frustration due to kagome-net arrangements include the weberite and pyrochlore ,,− lattices. These structures mainly consist of triangular or tetrahedral arrangements that impose spin constraints which cause magnetic frustration.…”