Beryllium fluoride is the inorganic compound with the formula BeF<sub>2</sub>. This white solid is the principal precursor for the manufacture of beryllium metal. Its structure resembles that of quartz, but BeF<sub>2</sub> is highly soluble in water.
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 à10<sup>âÂÂ14</sup>.
The structure of solid BeF<sub>2</sub> resembles that of cristobalite. Be<sup>2+</sup> centers are four coordinate and tetrahedral and the fluoride centers are two-coordinate. The Be-F bond lengths are about 1.54 ÃÂ . Analogous to SiO<sub>2</sub>, BeF<sub>2</sub> can also adopt a number of related structures. An analogy also exists between BeF<sub>2</sub> and AlF<sub>3</sub>: both adopt extended structures at mild temperature.
Gaseous beryllium fluoride adopts a linear structure, with a Be-F distance of 143 pm. BeF<sub>2</sub> reaches a vapor pressure of 10 Pa at 686 ðC, 100 Pa at 767 ðC, 1 kPa at 869 ðC, 10 kPa at 999 ðC, and 100 kPa at 1172 ðC. Molecular in the gaseous state is isoelectronic to carbon dioxide.
As a liquid, beryllium fluoride has a tetrahedral structure. The density of liquid BeF<sub>2</sub> decreases near its freezing point, as Be<sup>2+</sup> and F<sup>âÂÂ</sup> ions begin to coordinate more strongly with one another, leading to the expansion of voids between formula units.
The processing of beryllium ores generates impure Be(OH)<sub>2</sub>. This material reacts with ammonium bifluoride to give ammonium tetrafluoroberyllate:
Tetrafluoroberyllate is a robust ion, which allows its purification by precipitation of various impurities as their hydroxides. Heating purified (NH<sub>4</sub>)<sub>2</sub>BeF<sub>4</sub> gives the desired product:
In general the reactivity of BeF<sub>2</sub> ions with fluoride are quite analogous to the reactions of SiO<sub>2</sub> with oxides.
Reduction of BeF<sub>2</sub> at 1300 ðC with magnesium in a graphite crucible provides the most practical route to metallic beryllium:
Beryllium chloride is not a useful precursor because of its volatility.
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 to form a base solvent (FLiBe), into which fluorides of uranium and thorium are introduced. Beryllium fluoride is exceptionally chemically stable, and LiF/BeF<sub>2</sub> mixtures (FLiBe) have low melting points (360âÂÂ459 ðC) and the best neutronic properties of fluoride salt combinations appropriate for reactor use. MSRE used two different mixtures in the two cooling circuits.
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.