Beryllium fluoride is the inorganic compound with the formula Be F₂. This white solid is the principal precursor for the manufacture of

beryllium Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to form m ...

metal. Its structure resembles that of quartz, but BeF₂ is highly soluble in water.

Properties

Beryllium fluoride has distinctive optical properties. In the form of fluoroberyllate glass it has the lowest

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

for a solid at room temperature of 1.275. Its dispersive power is the lowest for a solid at 0.0093, and the

nonlinear coefficient Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typica ...

is also the lowest at 2 × 10⁻¹⁴.

Structure and bonding

The structure of solid BeF₂ resembles that of

cristobalite Cristobalite is a mineral polymorph of silica that is formed at very high temperatures. It has the same chemical formula as quartz, SiO2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the ...

. Be²⁺ centers are four coordinate and tetrahedral and the fluoride centers are two-coordinate. The Be-F bond lengths are about 1.54 Å. Analogous to SiO₂, BeF₂ can also adopt a number of related structures. An analogy also exists between BeF₂ and AlF₃: both adopt extended structures at mild temperature.

Gas and liquid BeF₂

Gaseous beryllium fluoride adopts a linear structure, with a Be-F distance of 143 pm. BeF₂ reaches a

vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phas ...

of 10 Pa at 686 °C, 100 Pa at 767 °C, 1 kPa at 869 °C, 10 kPa at 999 °C, and 100 kPa at 1172 °C. 'Molecules' of liquid beryllium fluoride have a fluctuating

tetrahedral In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the ...

structure. Additionally, the density of liquid BeF₂ decreases near its freezing point, as Be²⁺ and F⁻ ions begin to coordinate more strongly with one another, leading to the expansion of voids between

formula unit In chemistry, a formula unit is the empirical formula of any ionic or covalent network solid compound used as an independent entity for stoichiometric calculations. It is the lowest whole number ratio of ions represented in an ionic compound. E ...

Production

The processing of beryllium ores generates impure Be(OH)₂. This material reacts with

ammonium bifluoride Ammonium hydrogen fluoride is the inorganic compound with the formula or . It is produced from ammonia and hydrogen fluoride. This colourless salt is a glass- etchant and an intermediate in a once-contemplated route to hydrofluoric acid. Structu ...

to give ammonium tetrafluoroberyllate: :Be(OH)₂ + 2 (NH₄)HF₂ → (NH₄)₂BeF₄ + 2 H₂O Tetrafluoroberyllate is a robust ion, which allows its purification by precipitation of various impurities as their hydroxides. Heating purified (NH₄)₂BeF₄ gives the desired product: :(NH₄)₂BeF₄ → 2 NH₃ + 2 HF + BeF₂ In general the reactivity of BeF₂ ions with fluoride are quite analogous to the reactions of SiO₂ with oxides.

Applications

Reduction of BeF₂ at 1300 °C with magnesium in a

graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on lar ...

crucible A crucible is a ceramic or metal container in which metals or other substances may be melted or subjected to very high temperatures. While crucibles were historically usually made from clay, they can be made from any material that withstands te ...

provides the most practical route to metallic beryllium:Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. . :BeF₂ + Mg → Be + MgF₂ The chloride is not a useful precursor because of its volatility.

Niche uses

Beryllium fluoride is used in biochemistry, particularly protein crystallography as a mimic of phosphate. Thus, ADP and beryllium fluoride together tend to bind to ATP sites and inhibit protein action, making it possible to crystallise proteins in the bound state. Beryllium fluoride forms a basic constituent of the preferred fluoride salt mixture used in liquid-fluoride nuclear reactors. Typically beryllium fluoride is mixed with

lithium fluoride Lithium fluoride is an inorganic compound with the chemical formula LiF. It is a colorless solid, that transitions to white with decreasing crystal size. Although odorless, lithium fluoride has a bitter-saline taste. Its structure is analogous to ...

to form a base solvent (

FLiBe FLiBe is a molten salt made from a mixture of lithium fluoride (LiF) and beryllium fluoride (BeF2). It is both a nuclear reactor coolant and solvent for fertile or fissile material. It served both purposes in the Molten-Salt Reactor Experim ...

), into which fluorides of uranium and thorium are introduced. Beryllium fluoride is exceptionally chemically stable, and LiF/BeF₂ mixtures (

) have low melting points (360–459 °C) and the best neutronic properties of fluoride salt combinations appropriate for reactor use. MSRE used two different mixtures in the two cooling circuits.

Safety

Beryllium compounds are highly toxic. The increased toxicity of beryllium in the presence of fluoride has been noted as early as 1949. The in mice is about 100 mg/kg by ingestion and 1.8 mg/kg by intravenous injection.

References

External links

IARC Monograph "Beryllium and Beryllium Compounds"

* ttps://web.archive.org/web/20060116134617/http://www.npi.gov.au/database/substance-info/profiles/44.html National Pollutant Inventory: Fluoride and compounds fact sheet
Hazards of Beryllium fluoride

{{DEFAULTSORT:Beryllium Fluoride Beryllium compounds Fluorides Alkaline earth metal halides