Tetrafluoroberyllate
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Tetrafluoroberyllate or orthofluoroberyllate is an
anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
containing
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 ...
and fluorine. The fluoroanion has a tetrahedral shape, with the four fluorine atoms surrounding a central beryllium atom. It has the same size and outer electron structure as
sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
. Therefore, many compounds that contain sulfate have equivalents with tetrafluoroberyllate. Examples of these are the langbeinites, and Tutton's salts.


Properties

The Be–F
bond length In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of ...
is between 145 and 153 pm. The beryllium has sp3 atomic hybridization, leading to a longer bond than in BeF2, where the Be is sp hybridized. 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 temperature, they are hindered from moving. But as temperature increases, they can rotate around the threefold axis, (ie a line through one fluorine atom and the beryllium atom) with a potential barrier of . At higher temperatures, the movement can become isotropic (not limited to rotation on one axis) with a potential barrier of . Similar formula compounds have magnesium or zinc in a similar position to beryllium, e.g. K2MgF4 ( magnesium tetrafluoride) or (NH4)2ZnF4 ( tetrafluorozincate) but these are not as stable. Tetrafluoroberyllate has a biological effect by inhibiting
F-ATPase F-ATPase, also known as F-Type ATPase, is an ATPase/ synthase found in bacterial plasma membranes, in mitochondrial inner membranes (in oxidative phosphorylation, where it is known as Complex V), and in chloroplast thylakoid membranes. It uses ...
ATP producing enzymes in mitochondria 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.


Simple salts

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 °C and 320 °C it is in the α′ form. When temperature is raised above 320 °C it changes to the hexagonal α form. When cooled the α′ form changes to β form at 110 °C and this can be cooled to 70 °C before changing back to the γ form. It can be formed by melting
sodium fluoride Sodium fluoride (NaF) is an inorganic compound with the formula . It is used in trace amounts in the fluoridation of drinking water, in toothpaste, in metallurgy, and as a flux. It is a colorless or white solid that is readily soluble in water. I ...
and
beryllium fluoride Beryllium fluoride is the inorganic compound with the formula Be 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 B ...
. 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 A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. Only two MSRs have ever operated, both research reactors in the United States. The 1950's ...
, in which it is called
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 ...
. 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. Potassium tetrafluoroberyllate has the same structure as anhydrous
potassium sulfate Potassium sulfate (US) or potassium sulphate (UK), also called sulphate of potash (SOP), arcanite, or archaically potash of sulfur, is the inorganic compound with formula K2SO4, a white water-soluble solid. It is commonly used in fertilizers, prov ...
, as does rubidium and caesium tetrafluoroberyllate. Potassium tetrafluoroberyllate can make solid solutions with potassium sulfate. It can be used as a starting point to make the non-linear optic crystal KBe2BO3F2 which has the highest power handling capacity and shortest UV performance of any borate. It is quite soluble in water, so beryllium can be extracted from soil in this form. Ammonium tetrafluoroberyllate decomposes on heating by losing NH4F vapour, progressively forming NH4BeF3, then NH4Be2F5 and finally BeF2. Thallium tetrafluoroberyllate can be made by dissolving
beryllium fluoride Beryllium fluoride is the inorganic compound with the formula Be 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 B ...
and thallium carbonate together in hydrofluoric acid and then evaporating the solution. Radium tetrafluoroberyllate is used as a standard neutron source. The
alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be pr ...
s from the
radium Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rathe ...
cause
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
s to be emitted from the beryllium. It is precipitated from a
radium chloride Radium chloride (RaCl2) is a salt of radium and chlorine, and the first radium compound isolated in a pure state. Marie Curie and André-Louis Debierne used it in their original separation of radium from barium. The first preparation of radium me ...
solution mixed with potassium tetrafluoroberyllate. Magnesium tetrafluoroberyllate can be precipitated from a hot saturated solution of ammonium tetrafluoroberyllate and a magnesium salt. However, if the temperature reaches boiling point MgF2 is precipitated instead. Calcium tetrafluoroberyllate resembles
zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of t ...
in the way it melts and crystallises. Strontium tetrafluoroberyllate can be made in several forms. The γ form is produced by cooling a melt of SrF2 and Be2 and the β form is made by precipitating from a water solution. When melted and heated to 850–1145 °C, Be2 gas evaporates leaving behind molten SrF2. The barium tetrafluoroberyllate is very insoluble and can be used for
gravimetric analysis Gravimetric analysis describes a set of methods used in analytical chemistry for the quantitative determination of an analyte (the ion being analyzed) based on its mass. The principle of this type of analysis is that once an ion's mass has been ...
of beryllium. H2BeF4 is an acid that can be produced from Ag2BeF4 and HCl. It only exists in aqueous solution. Triglycine tetrafluoroberyllate (TGFB) is
ferroelectric Ferroelectricity is a characteristic of certain materials that have a spontaneous electric polarization that can be reversed by the application of an external electric field. All ferroelectrics are also piezoelectric and pyroelectric, with the ad ...
with a transition point of 70 °C. The crystals can be formed by dissolving BeF2 in water, adding HF and then glycine. When the solution is cooled triglycine tetrafluoroberyllate forms. Cs2BeF4 and Tl2BeF4 in the solution reduce growth on the 001 direction so that tabular shaped crystals of TGFB form. The thallium compound can cut growth on the 001 axis by 99%.


Double salts


Tuttons salts

The Tuttons salt (NH4)2Mn(BeF4)2·6(H2O) is made from a solution of NH4BeF3 mixed with NH4MnF3. The equivalent of alums are hard to make because the trivalent ion will often form a complex with fluoride in preference to the beryllium fluoride. However the violet coloured acid and rubidium chrome alum exist at chilly temperatures for a few hours. Tutton's salts (also called schoenites) containing magnesium with fluoroberyllate are difficult to produce, as the solutions tend to precipitate insoluble MgF2.


Alums

Tetrafluoroberyllate salts equivalent to
alums An alum () is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula , where is a monovalent cation such as potassium or ammonium. By itself, "alum" often refers to potassium alum, with the f ...
also exist with formula MABF4·12H2O, where M is univalent, and A trivalent. These are not common as fluoride often form insoluble products with the trivalent ions. Methods to produce these include evaporating mixed fluoride solutions under reduced pressure at 0 °C, or dissolving beryllium and other metal hydroxides in hydrofluoric acid at room temperature, cooled, and them mixing with cold ethyl alcohol, causing cooling and crystallisation. The unit cell dimensions are slightly smaller (by 0.03–0.05 Å) than the corresponding sulfate alums.


References

{{fluorine compounds Beryllium compounds Fluorine compounds Anions Fluorometallates