A Geometrically Frustrated Family of M^IIM^IIIF₅(H₂O)₂ Mixed–Metal Fluorides with Complex Magnetic Interactions

Abstract

Inverse weberites are of interest as geometrically frustrated magnetic materials due to their unique cation arrangement. We have synthesized nine isostructural materials that adopt the inverse weberite crystal structure, which consists of cross-linked kagome layers. These materials, having the general formula M II M III F 5 (H 2 O) 2 (M II = Co, Mn, Ni, Zn; M III = Ga, Cr, Fe, V), were synthesized using mild hydrothermal conditions, which yielded phase-pure samples after optimization of the reaction conditions. Their crystal structures and optical, thermal, and magnetic behavior were characterized using single-crystal X-ray diffraction, UV−vis spectroscopy, thermogravimetric analysis, and measurement of the magnetic susceptibility and isothermal magnetization data, respectively. Three distinct types of magnetism were observed, including simple paramagnetism, antiferromagnetism, and canted antiferromagnetism; the last type is accompanied by a high frustration index f in the range 4.16−8.09. We demonstrated that the magnetic behavior of inverse weberites depends on the presence or absence of unpaired-electron-containing cations on the two distinct crystallographic sites, which can be employed for the prediction of the magnetic properties of other compounds in this rich and diverse family.

Comparative Study on Crystal Structures and Synthetic Techniques of Ternary Hafnium/Zirconium Fluorides

“…40−43 Here, the highly reactive hydrofluoric acid acts as a mineralizer as well as a fluorinating reagent, and HNO 3 lowers the pH. 23,34 These reaction conditions, optimized via an Edisonian approach, enable the fluorination of the stable HfO 2 and allow the formation of ternary fluoride crystals at such low temperatures. The use of different divalent cations, Mg, Ca, Sr, Ba, Pb, and Cd, likely for size reasons, did not result in an isotypic series but rather resulted in a variety of different crystal structures reported herein (Figure 1).…”

“…Mild hydrothermal synthesis was utilized to obtain a series of ternary hafnium fluorides consisting of high-quality single crystals suitable for single-crystal X-ray diffraction. The synthesis was carried out at a very low temperature of 160 °C, in contrast to the very high temperatures (>700 °C) used in typical solid-state ternary hafnium-containing fluoride synthesis. − Here, the highly reactive hydrofluoric acid acts as a mineralizer as well as a fluorinating reagent, and HNO 3 lowers the pH. , These reaction conditions, optimized via an Edisonian approach, enable the fluorination of the stable HfO 2 and allow the formation of ternary fluoride crystals at such low temperatures. The use of different divalent cations, Mg, Ca, Sr, Ba, Pb, and Cd, likely for size reasons, did not result in an isotypic series but rather resulted in a variety of different crystal structures reported herein (Figure ).…”

“…In our group, we have used both molten salt synthesis − and mild hydrothermal synthesis − for the growth of single crystals of a large variety of fluoride phases. For the hydrothermal route using aqueous HF, we established that mild conditions, 160–200 °C and relatively short reaction times, 24 h, followed by slow cooling to room temperature, often yielded high-quality crystals of ternary and quaternary fluorides whose structures could readily be determined.…”

“…For the hydrothermal route using aqueous HF, we established that mild conditions, 160–200 °C and relatively short reaction times, 24 h, followed by slow cooling to room temperature, often yielded high-quality crystals of ternary and quaternary fluorides whose structures could readily be determined. We have used this approach to explore, in general, the preparation of transition metal- and rare earth-containing fluorides and oxyfluorides, including vanadates, niobates, tantalates, tungstates, molybdates, manganates, cobaltates, nickelates, and cerates, as well as numerous actinide-containing fluoride phases containing Th, U, Np, Pu, and Am. ,,− ,− During this process, by experience, we have learned much about how to choose the optimum reaction conditions. In recent work, we explored hafnium fluorides, which yielded a number of hydrated phases, including Cs 2 [M­(H 2 O) 6 ]­[Hf 2 F 12 ] (M = Ni, Co, and Zn) and CuHfF 6 (H 2 O) 4 .…”

Investigation of Charge-Ordered Barium Iron Fluorides with One-Dimensional Structural Diversity and Complex Magnetic Interactions

“…20 More commonly, these mixed valence compounds can be extended to mixed-metal fluorides when one of the identical metal cations is replaced by different cations with the same oxidation states, such as mixed-metal fluorides AM II M III F 6 [A = alkali metal or NH 4 + ; M(II) = Mg, Mn, Fe, Co, Ni, Cu; and M(III) = Al, Ga, V, Cr, Fe] 21,22 developed from defect pyrochlore fluorites of the type AB II B III F 6 . Recently, a geometrically frustrated family of M II M III F 5 (H 2 O) 2 (M II = Co, Mn, Ni, Zn; M III = Ga, Cr, Fe, V) 23 The synthesis of fluoride materials from ceramic routes generally requires HF, F 2 , and CF 4 gases as fluorination sources. Due to the lack of fluorination facilities for their safe handling, 24,25 this method has been progressively abandoned.…”