Beryllium fluoride is the inorganic compound with the chemical formula, formula Beryllium, BeFluorine, F₂. This white solid is the principal precursor for the manufacture of beryllium 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 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 is also the lowest at 2 × 10⁻¹⁴.

Structure and bonding

The structure of solid BeF₂ resembles that of cristobalite. 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 silicon dioxide, 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 Picometre, pm. BeF₂ reaches a vapor pressure of 10 Pascal (unit), 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 molecular geometry, tetrahedral structure. Additionally, the density of liquid BeF₂ decreases near its freezing point, as Be²⁺ and fluoride, F⁻ ions begin to coordinate more strongly with one another, leading to the expansion of voids between formula units.

Production

The processing of beryllium ores generates impure Be(OH)₂. This material reacts with ammonium bifluoride 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 crucible 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, adenosine diphosphate, ADP and beryllium fluoride together tend to bind to adenosine triphosphate, 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 reactor, liquid-fluoride nuclear reactors. Typically beryllium fluoride is mixed with lithium fluoride to form a base solvent (FLiBe), into which fluorides of uranium and thorium are introduced. Beryllium fluoride is exceptionally chemically stable, and LiF/BeF₂ mixtures (FLiBe) have low melting points (360–459 °C) and the best neutronic properties of fluoride salt combinations appropriate for reactor use. Molten-Salt Reactor Experiment, 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"

* [https://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