Iron(II) fluoride or ferrous fluoride is an

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

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.Dale L. Perry (1995),
Handbook of Inorganic Compounds
, page 167. CRC Press.

Structure and bonding

Anhydrous FeF₂ adopts the TiO₂

rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest refractive indices at vis ...

structure. As such, the iron cations are octahedral and fluoride anions are trigonal planar. The tetrahydrate can exist in two structures, or polymorphs. One form is rhombohedral and the other is hexagonal, the former having a disorder. Like most fluoride compounds, the anhydrous and hydrated forms of iron(II) fluoride feature high spin metal center. Low temperature

neutron diffraction Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material. A sample to be examined is placed in a beam of Neutron temperature, thermal or ...

studies show that the FeF₂ is antiferromagnetic.

Heat capacity Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K). Heat capacity is a ...

measurements reveal an event at 78.3 K corresponding to ordering of antiferromagnetic state.

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. The following reaction is proposed in order to calculate the atomization energy for Fe⁺: :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. It is slightly soluble in water (with

solubility product Solubility equilibrium is a type of dynamic equilibrium that exists when a chemical compound in the solid state is in chemical equilibrium with a solution of that compound. The solid may dissolve unchanged, with dissociation, or with chemical reac ...

K_sp = 2.36×10⁻⁶ at 25 °C) as well as dilute

hydrofluoric acid Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colorless, acidic and highly corrosive. A common concentration is 49% (48–52%) but there are also stronger solutions (e.g. 70%) and pure HF has a boiling p ...

, giving a pale green solution. It is insoluble in organic solvents. The tetrahydrate can be prepared by dissolving iron in warm hydrated

and precipitating the result by addition of

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

. It oxidizes in moist air to give, inter alia, a hydrate of

iron(III) fluoride Iron(III) fluoride, also known as ferric fluoride, are inorganic compounds with the formula FeF3(H2O)x where x = 0 or 3. They are mainly of interest by researchers, unlike the related iron(III) chloride. Anhydrous iron(III) fluoride is white, whe ...

, (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, FeF_X (x = 2, 3) operates via a complex conversion mechanism, resulting in higher energy density. Fluoride cathodes are stable up to 1000°C. Stability not only enhances safety and lowers the risk of thermal runaway. FeF_X exhibits distinctive phase evolution, intermediate phases, and morphological transformations during lithiation and delithiation. A stable lattice of fluoride anions is maintained throughout charge and discharge cycles, consistent with high cycling reversibility.

References

External links

National Pollutant Inventory - Fluoride and compounds fact sheet
{{fluorides Fluorides Metal halides Iron(II) compounds