Iron(II) fluoride explained

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]

Structure and bonding

Anhydrous FeF2 adopts the TiO2 rutile structure. As such, the iron cations are octahedral and fluoride anions are trigonal planar.[2] [3]

The tetrahydrate can exist in two structures, or polymorphs. One form is rhombohedral and the other is hexagonal, the former having a disorder.[4]

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.[5] Heat capacity measurements reveal an event at 78.3 K corresponding to ordering of antiferromagnetic state.[6]

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.[7]

The following reaction is proposed in order to calculate the atomization energy for Fe+:[8]

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.[9] It is slightly soluble in water (with solubility product Ksp = 2.36×10−6 at 25 °C)[10] as well as dilute hydrofluoric acid, giving a pale green solution.[4] It is insoluble in organic solvents.

The tetrahydrate can be prepared by dissolving iron in warm hydrated hydrofluoric acid and precipitating the result by addition of ethanol.[4] It oxidizes in moist air to give, inter alia, a hydrate of iron(III) fluoride, (FeF3)2·9H2O.

Uses

FeF2 is used to catalyze some organic reactions.

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, FeFX (x = 2, 3) operates via a complex conversion mechanism, resulting in higher energy density. Fluoride cathodes are stable up to 1000°C.[11] Stability not only enhances safety and lowers the risk of thermal runaway.[12]

FeFX exhibits distinctive phase evolution, intermediate phases, and morphological transformations during lithiation and delithiation.[13] [14] A stable lattice of fluoride anions is maintained throughout charge and discharge cycles, consistent with high cycling reversibility.[15] [16]

External links

Notes and References

  1. Dale L. Perry (1995), "Handbook of Inorganic Compounds", page 167. CRC Press.
  2. Stout. J.. Stanley A. Reed. The Crystal Structure of MnF2, FeF2, CoF2, NiF2 and ZnF2. J. Am. Chem. Soc.. 1954. 76 . 21. 5279–5281. 10.1021/ja01650a005.
  3. M.J.M.. de Almeida. M.M.R.. Costa. J.A.. Paixão. 1989-12-01. Charge density of FeF2. Acta Crystallographica Section B. en. 45. 6. 549–555. 10.1107/S0108768189007664. 0108-7681.
  4. 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 . 10.1107/S0365110X60002302 . free .
  5. Erickson. R. . Neutron Diffraction Studies of Antiferromagnetism in Manganous Fluoride and Some Isomorphous Compounds. Physical Review. June 1953. 90. 5. 779–785. 10.1103/PhysRev.90.779. 1953PhRv...90..779E .
  6. Stout . J. . Edward Catalano . Thermal Anomalies Associated with the Antiferromagnetic Ordering of FeF2, CoF3, and NiF2. Physical Review. December 1953. 92. 6. 1575. 10.1103/PhysRev.92.1575. 1953PhRv...92.1575S .
  7. 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. 10.1021/je950115w. 0021-9568.
  8. Kent. Richard. 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. 10.1021/ja00949a016. John L. Margrave.
  9. 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.
  10. Web site: SOLUBILITY PRODUCT CONSTANTS. 2016-11-07. 2018-07-12. https://web.archive.org/web/20180712140117/http://www4.ncsu.edu/~franzen/public_html/CH201/data/Solubility_Product_Constants.pdf. dead.
  11. 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 . en . 66 . 1 . 89–95 . 10.1134/S0036023621010058 . 1531-8613.
  12. 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 . en . 603 . 7903 . E30 . 10.1038/s41586-022-04612-5 . 35293393 . 1476-4687.
  13. Badway . F. . Cosandey . F. . Pereira . N. . Amatucci . G. G. . 2003 . Carbon Metal Fluoride Nanocomposites . Journal of the Electrochemical Society . en . 150 . 10 . A1318 . 10.1149/1.1602454.
  14. 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 . 10.1016/j.coelec.2021.100779 . 2451-9103.
  15. 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 . en . 19 . 6 . 644–654 . 10.1038/s41563-020-0621-z . 32094491 . 1476-4660.
  16. 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 . en . 5 . 2 . 101787 . 10.1016/j.xcrp.2024.101787.

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