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Iron(II) fluoride
Identifiers
CAS Number	7789-28-8 anhydrous ; 13940-89-1 (tetrahydrate) ;
3D model (JSmol)	Interactive image;
ChemSpider	74215 ;
ECHA InfoCard	100.029.232
PubChem CID	522690;
UNII	NP4W87HLVO ;
CompTox Dashboard (EPA)	DTXSID50894779 ;
	InChI InChI=1S/2FH.Fe/h21H;/q;;+2/p-2 Key: FZGIHSNZYGFUGM-UHFFFAOYSA-L ; InChI=1/2FH.Fe/h21H;/q;;+2/p-2 Key: FZGIHSNZYGFUGM-NUQVWONBAX;
	SMILES [Fe+2].[F-].[F-];
Properties
Chemical formula	FeF2
Molar mass	93.84 g/mol (anhydrous) ; 165.902 g/mol (tetrahydrate)
Appearance	colorless transparent crystals
Density	4.09 g/cm3 (anhydrous) ; 2.20 g/cm3 (tetrahydrate)
Melting point	970 °C (1,780 °F; 1,240 K) (anhydrous) ; 100 °C (tetrahydrate)
Boiling point	1,100 °C (2,010 °F; 1,370 K) (anhydrous)
Solubility product (Ksp)	2.36×10−6
Solubility	insoluble in ethanol, ether; ; dissolves in HF
Magnetic susceptibility (χ)	+9500.0·10−6 cm3/mol
Structure
Crystal structure	Rutile (tetragonal), tP6
Space group	P42/mnm, No. 136
Hazards
	Occupational safety and health (OHS/OSH):
Main hazards	Causes severe skin burns & eye damage; ; Hazardous decomposition products formed under fire conditions- Iron oxides
	GHS labelling:
Pictograms
NFPA 704 (fire diamond)	4 0 2 COR
Flash point	not applicable
Related compounds
Other anions	Iron(II) chloride; Iron(II) bromide; Iron(II) iodide; Iron(II) oxide
Other cations	Manganese(II) fluoride; Cobalt(II) fluoride
Related compounds	Iron(III) fluoride
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

Iron(II) fluoride or ferrous fluoride is an inorganic compound with the molecular formula FeF₂. It forms a tetrahydrate FeF₂·4H₂O that is often referred to by the same names. The anhydrous and hydrated forms are white crystalline solids.^[1]^[5]

Structure and bonding

Anhydrous FeF₂ adopts the TiO₂ rutile structure. As such, the iron cations are octahedral and fluoride anions are trigonal planar.^[6]^[7]

The tetrahydrate can exist in two structures, or polymorphs. One form is rhombohedral and the other is hexagonal, the former having a disorder.^[1]

Like most fluoride compounds, the anhydrous and hydrated forms of iron(II) fluoride feature high spin metal center. Low temperature neutron diffraction studies show that the FeF₂ is antiferromagnetic.^[8] Heat capacity measurements reveal an event at 78.3 K corresponding to ordering of antiferromagnetic state.^[9]

Selected physical properties

FeF₂ sublimes between 958 and 1178 K. Using Torsion and Knudsen methods, the heat of sublimation was experimentally determined and averaged to be 271 ± 2 kJ mole⁻¹.^[10]

The following reaction is proposed in order to calculate the atomization energy for Fe⁺:^[11]

FeF₂ + e → Fe⁺ + F₂ (or 2F) + 2e

Synthesis and reactions

The anhydrous salt can be prepared by reaction of ferrous chloride with anhydrous hydrogen fluoride.^[12] It is slightly soluble in water (with solubility product K_sp = 2.36×10⁻⁶ at 25 °C)^[13] as well as dilute hydrofluoric acid, giving a pale green solution.^[1] It is insoluble in organic solvents.^[5]

The tetrahydrate can be prepared by dissolving iron in warm hydrated hydrofluoric acid and precipitating the result by addition of ethanol.^[1] It oxidizes in moist air to give, inter alia, a hydrate of iron(III) fluoride, (FeF₃)₂·9H₂O.^[1]

Uses

FeF₂ is used to catalyze some organic reactions.^[14]

Battery research

FeF₂ has been investigated as a cathode material for both lithium-ion and fluoride-ion batteries. Unlike conventional metal oxides, which rely on an intercalation-based lithium storage mechanism, FeF_X (x = 2, 3) operates via a complex conversion mechanism, resulting in higher energy density. Fluoride cathodes are stable up to 1000°C.^[15] Stability not only enhances safety and lowers the risk of thermal runaway.^[16]

FeF_X exhibits distinctive phase evolution, intermediate phases, and morphological transformations during lithiation and delithiation.^[17]^[18] A stable lattice of fluoride anions is maintained throughout charge and discharge cycles, consistent with high cycling reversibility.^[19]^[20]

References

^ ^a ^b ^c ^d ^e ^f Penfold, B. R.; Taylor, M. R. (1960). "The crystal structure of a disordered form of iron(II) fluoride tetrahydrate". Acta Crystallographica. 13 (11): 953–956. doi:10.1107/S0365110X60002302.
^ John Rumble (June 18, 2018). CRC Handbook of Chemistry and Physics (99 ed.). CRC Press. pp. 5–188. ISBN 978-1138561632.
^ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
^ ^a ^b Sigma-Aldrich. "Material Safety Data Sheet". Sigma-Aldrich. Retrieved 5 April 2011.
^ ^a ^b Dale L. Perry (1995), "Handbook of Inorganic Compounds", page 167. CRC Press. ISBN 9780849386718
^ Stout, J.; Stanley A. Reed (1954). "The Crystal Structure of MnF₂, FeF₂, CoF₂, NiF₂ and ZnF₂". J. Am. Chem. Soc. 76 (21): 5279–5281. doi:10.1021/ja01650a005.
^ M.J.M., de Almeida; M.M.R., Costa; J.A., Paixão (1989-12-01). "Charge density of FeF2". Acta Crystallographica Section B. 45 (6): 549–555. doi:10.1107/S0108768189007664. ISSN 0108-7681.
^ Erickson, R. (June 1953). "Neutron Diffraction Studies of Antiferromagnetism in Manganous Fluoride and Some Isomorphous Compounds". Physical Review. 90 (5): 779–785. Bibcode:1953PhRv...90..779E. doi:10.1103/PhysRev.90.779.
^ Stout, J.; Edward Catalano (December 1953). "Thermal Anomalies Associated with the Antiferromagnetic Ordering of FeF₂, CoF₃, and NiF₂". Physical Review. 92 (6): 1575. Bibcode:1953PhRv...92.1575S. doi:10.1103/PhysRev.92.1575.
^ Bardi, Gianpiero; Brunetti, Bruno; Piacente, Vincenzo (1996-01-01). "Vapor Pressure and Standard Enthalpies of Sublimation of Iron Difluoride, Iron Dichloride, and Iron Dibromide". Journal of Chemical & Engineering Data. 41 (1): 14–20. doi:10.1021/je950115w. ISSN 0021-9568.
^ Kent, Richard; John L. Margrave (November 1965). "Mass Spectrometric Studies at High Temperatures. VIII. The Sublimation Pressure of Iron(II) Fluoride". Journal of the American Chemical Society. 87 (21): 4754–4756. doi:10.1021/ja00949a016.
^ W. Kwasnik "Iron(II) Fluoride" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 266.
^ "SOLUBILITY PRODUCT CONSTANTS" (PDF). Archived from the original (PDF) on 2018-07-12. Retrieved 2016-11-07.
^ Wildermuth, Egon; Stark, Hans; Friedrich, Gabriele; Ebenhöch, Franz Ludwig; Kühborth, Brigitte; Silver, Jack; Rituper, Rafael (2000). "Iron Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a14_591. ISBN 978-3527306732.
^ Nikitin, M. I.; Chilingarov, N. S.; Alikhanyan, A. S. (2021-01-01). "Thermal Stability of Mixed Fluorides of 3d Elements". Russian Journal of Inorganic Chemistry. 66 (1): 89–95. doi:10.1134/S0036023621010058. ISSN 1531-8613.
^ Viswanathan, Venkatasubramanian; Epstein, Alan H.; Chiang, Yet-Ming; Takeuchi, Esther; Bradley, Marty; Langford, John; Winter, Michael (March 2022). "Author Correction: The challenges and opportunities of battery-powered flight". Nature. 603 (7903): E30. doi:10.1038/s41586-022-04612-5. ISSN 1476-4687. PMID 35293393.
^ Badway, F.; Cosandey, F.; Pereira, N.; Amatucci, G. G. (2003). "Carbon Metal Fluoride Nanocomposites". Journal of the Electrochemical Society. 150 (10): A1318. doi:10.1149/1.1602454.
^ Olbrich, Lorenz F.; Xiao, Albert W.; Pasta, Mauro (2021-12-01). "Conversion-type fluoride cathodes: Current state of the art". Current Opinion in Electrochemistry. 30: 100779. doi:10.1016/j.coelec.2021.100779. ISSN 2451-9103.
^ Xiao, Albert W.; Lee, Hyeon Jeong; Capone, Isaac; Robertson, Alex; Wi, Tae-Ung; Fawdon, Jack; Wheeler, Samuel; Lee, Hyun-Wook; Grobert, Nicole; Pasta, Mauro (June 2020). "Understanding the conversion mechanism and performance of monodisperse FeF2 nanocrystal cathodes". Nature Materials. 19 (6): 644–654. doi:10.1038/s41563-020-0621-z. ISSN 1476-4660. PMID 32094491.
^ Olbrich, Lorenz F.; Xiao, Albert W.; Schart, Maximilian; Ihli, Johannes; Matthews, Guillaume; Sanghadasa, Mohan; Pasta, Mauro (February 2024). "Iron fluoride-lithium metal batteries in bis(fluorosulfonyl)imide-based ionic liquid electrolytes". Cell Reports Physical Science. 5 (2): 101787. doi:10.1016/j.xcrp.2024.101787.