Tetrafluoroberyllate
Tetrafluoroberyllate or orthofluoroberyllate BeF42− is an anion containing beryllium and fluorine. The ion has a tetrahedral shape, the same size and outer electron structure as sulfate. Therefore, many compounds that contain sulfate, have equivalents with tetrafluoroberyllate. Examples of these are the Langbeinites, and Tutton's salts.
The Be–F distance is 1.45-1.53Å. This bond is sp3 and has a longer length than the sp bond in BeF2 gas. In trifluoroberyllates, there are actually BeF4 tetrahedra arranged in a triangle, so that three fluorine atoms are shared on two tetrahedra each, resulting in a formula of Be3F9.
In the tetrafluoroberyllates the tetrahedra can rotate to various degrees. At room temperatures they are hindered from moving. But as temperature increases they can rotate around the three-fold access, with a potential barrier of 12.5 kcal/mol. At higher temperatures the movement can become isotropic with a potential barrier of 14.5 kcal/mol.
Similar formula compounds have magnesium or zinc in a similar position. e.g. K2MgF4 or (NH4)2ZnF4 but these are not as stable.
Tetrafluoroberyllate has a biological effect by inhibiting F-ATPase ATP producing enzymes in and bacteria. It does this by attempting to react with adenosine diphosphate because it resembles phosphate. However once it does this it remains stuck in the F1 part of the enzyme and inhibits it from further function.
Sodium tetrafluoroberyllate has several crystalline forms. Below 220 °C it takes the same form as orthorhombic olivine, and this is called γ phase. Between 220 and 320 it is in the α' form. When temperature is raised above 320 it changes to the hexagonal α form. When cooled the α' form changes to β form at 110° and this can be cooled to 70° before changing back to the γ form. It can be formed by melting sodium fluoride and beryllium fluoride. The gas above molten sodium tetrafluoroberyllate contains BeF2 and NaF gas.
Lithium tetrafluoroberyllate takes on the same crystal form as the mineral phenacite. As a liquid it is proposed for the molten salt reactor, in which it is called FLiBe. The liquid salt has a high specific heat, similar to that of water. The molten salt has a very similar density to the solid. The solid has continuous void channels through it, which reduces its density. Li2BeF4 can be crystallised from aqueous solution using (NH4)2BeF4 and LiCl.
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