# Tetrafluoroberyllate

Tetrafluoroberyllate or orthofluoroberyllate is an anion with the chemical formula [BeF$_4$]$^{2−}$. It contains beryllium and fluorine. This fluoroanion has a tetrahedral shape, with the four fluorine atoms surrounding a central beryllium atom. It has the same size, charge, and outer electron structure as sulfate SO2−4. 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 is between 145 and 153 pm. The beryllium is sp$^3$ hybridized, leading to a longer bond than in BeF$_2$, where beryllium is sp hybridized. In trifluoroberyllates, there are actually BeF$_4$ tetrahedra arranged in a triangle, so that three fluorine atoms are shared on two tetrahedra each, resulting in a formula of Be$_3$F$_9$.
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, (i.e. a line through one fluorine atom and the beryllium atom) with a potential barrier of 12.5 kcal/mol (52 kJ/mol). At higher temperatures, the movement can become isotropic (not limited to rotation on one axis) with a potential barrier of 14.5 kcal/mol (61 kJ/mol).
Similar compounds have magnesium or zinc in a similar position as beryllium, e.g. K$_2$[MgF$_4$] (potassium tetrafluoromagnesate) or [NH$_4$]$_2$[ZnF$_4$] (ammonium tetrafluorozincate) but these are not as stable.
Tetrafluoroberyllate has a biological effect by inhibiting F-ATPase adenosine triphosphate 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

| Name | Chemical formula | Molar mass (g/mol) | CAS number | Crystal system | Density (g/cm$^3$) | Melting point (°C) | Solubility in water (g/(100 ml)) |
| lithium tetrafluoroberyllate | Li$_2$[BeF$_4$] | 98.89 | | | 2.167 | 472 °C | slight (1.25 at 20 °C, 5.78 at 40 °C) |
| sodium tetrafluoroberyllate | Na$_2$BeF$_4$ | 130.985333 | 13871-27-7 | Orthorhombic | 2.47 | 575 °C | slight (1.33 at 0 °C, 1.44 at 20 °C, 2.73 at 90 °C) |
| potassium tetrafluoroberyllate | K$_2$BeF$_4$ | 163.20 | 7787-50-0 | orthorhombic a = 5.691 Å, b = 7.278 Å, c = 9.896 Å as for strontium orthosilicate | 2.64 | | |
| potassium tetrafluoroberyllate dihydrate | K$_2$BeF$_4$·2H$_2$O | 199.233 | | | | |
| ammonium tetrafluoroberyllate | (NH$_4$)$_2$BeF$_4$ | 121.0827 | 14874-86-3 | orthorhombic a = 5.91 Å, b = 7.64 Å, c = 10.43 Å | 1.71 | decomposes 280 °C | 32.3 at 25 °C |
| rubidium tetrafluoroberyllate | Rb$_2$BeF$_4$ | 255.941 | | orthorhombic a = 5.87 Å, b = 7.649 Å, c = 10.184 Å | 3.72 | | |
| caesium tetrafluoroberyllate | Cs$_2$BeF$_4$ | 350.8167 | | orthorhomic a = 8.03 Å, b = 10.81 Å, c = 0.622 Å | 4.32 | | |
| thallium tetrafluoroberyllate | Tl$_2$BeF$_4$ | 493.7724 | | orthorhombic a = 7.7238 Å, b = 5.9022 Å, c = 10.4499 Å | 6.884 | | |
| silver tetrafluoroberyllate | Ag$_2$BeF$_4$ | 300.7422 | | | | | |
| magnesium tetrafluoroberyllate | MgBeF$_4$ | 109.3108 | | | | | |
| calcium tetrafluoroberyllate | CaBeF$_4$ | 125.08 | | | 2.959 | | |
| strontium tetrafluoroberyllate | SrBeF$_4$ | 172.6 | | orthorhombic a = 5.291 Å, b = 6.787 Å, c = 8.307 Å | 3.84 | | insoluble |
| barium tetrafluoroberyllate | BaBeF$_4$ | 222.333 | | | 4.17 | | insoluble |
| radium tetrafluoroberyllate | RaBeF$_4$ | 311.005795 | | | | | insoluble |
| hexaqua ferrous tetrafluoroberyllate | FeBeF$_4$·6H$_2$O | | | | | | |
| heptaqua ferrous tetrafluoroberyllate | FeBeF$_4$·7H$_2$O | | | | 1.894 | | |
| heptaqua nickel tetrafluoroberyllate | NiBeF$_4$·7H$_2$O | | | | |
| hexaqua nickel tetrafluoroberyllate | NiBeF$_4$·6H$_2$O | | | | | | |
| heptaqua cobalt tetrafluoroberyllate | CoBeF$_4$·7H$_2$O | | | | 1.867 | | |
| hexaqua cobalt tetrafluoroberyllate | CoBeF$_4$·6H$_2$O | | | | 1.891 | | |
| pentaqua copper tetrafluoroberyllate | CuBeF$_4$·5H$_2$O | | | | | | |
| heptaqua zinc tetrafluoroberyllate | ZnBeFe$_4$·7H$_2$O | | | | | | |
| lead tetrafluoroberyllate | PbBeF$_4$ | 292.2 | | | 6.135 | | |
| hydrazinium tetrafluoroberyllate | N$_2$H$_6$BeF$_4$ | 119.0668 | | a = 5.58 Å, b = 7.337 Å, c = 9.928 Å, α = 90°, β = 98.22°, γ = 90° | | | |
| triglycine tetrafluoroberyllate | (NH$_2$CH$_2$COOH)$_3$·H$_2$BeF$_4$ | 312.221 | 2396-72-7 | monoclinic | | | |
| ethylene diamine fluoroberyllate | (NH$_2$CH$_2$CH$_2$NH$_2$)·H$_2$BeF$_4$ | | | | | decomposes 330 °C | |
| propylenediamine tetrafluoroberyllate | (NH$_2$CH$_2$CH$_2$CH$_2$NH$_2$)·H$_2$BeF$_4$ | | | | | | |
| propylene-1,2-diamine tetrafluoroberyllate | (NH$_2$CH(CH$_3$)CH$_2$NH$_2$)·H$_2$BeF$_4$ | | | monoclinic a = 5.535 Å, b = 13.560 Å, c = 9.6048 Å, β = 106.73 Å, V = 690.4 Å$^3$, Z = 4 | 1.55 | | |
| benzidine fluoroberyllate | (NH$_2$C$_6$H$_4$C$_6$H$_4$NH$_2$)·H$_2$BeF$_4$ | | | | | | ins |
| tetramethyl ammonium tetrafluoroberyllate | [N(CH$_3$)$_4$]$_2$BeF$_4$ | | | | | | |
| tetramine silver tetrafluoroberyllate | [Ag(NH$_3$)$_2$]$_2$BeF$_4$ | | | | | | |
| | [Cu(NH$_3$)$_2$]$_2$BeF$_4$ | | | | | | |
| | [Cu(NH$_3$)$_4$]$_2$BeF$_4$·H$_2$O | | | | | | |
| | [Zn(NH$_3$)$_4$]$_2$BeF$_4$ | | | | | | |
| | [Cd(NH$_3$)$_4$]$_2$BeF$_4$ | | | | | | |
| | [Ni(NH$_3$)$_6$]$_2$BeF$_4$ | | | | | | |
| | [Ni(NH$_3$)$_4$]$_2$BeF$_4$·2H$_2$O | | | | | | |
| | [Ni(NH$_3$)$_2$]$_2$BeF$_4$ | | | | | | |
| | [Co(NH$_3$)$_6$]$_2$BeF$_4$·3H$_2$O | | | | | | |
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 and beryllium fluoride. The gas above molten sodium tetrafluoroberyllate contains BeF$_2$ 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. Li$_2$BeF$_4$ can be crystallised from aqueous solution using (NH$_4$)$_2$BeF$_4$ and LiCl.
Potassium tetrafluoroberyllate has the same structure as anhydrous potassium sulfate, 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 KBe$_2$BO$_3$F$_2$ 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 NH$_4$F vapour, progressively forming NH$_4$BeF$_3$, then NH$_4$Be$_2$F$_5$ and finally BeF$_2$.
Thallium tetrafluoroberyllate can be made by dissolving beryllium fluoride and thallium carbonate together in hydrofluoric acid and then evaporating the solution.
Radium tetrafluoroberyllate is used as a standard neutron source. The alpha particles from the radium cause neutrons to be emitted from the beryllium. It is precipitated from a radium chloride 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 MgF$_2$ is precipitated instead.
Calcium tetrafluoroberyllate resembles zircon 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 SrF$_2$ and Be$_2$ and the β form is made by precipitating from a water solution. When melted and heated to 850–1145 °C, Be$_2$ gas evaporates leaving behind molten SrF$_2$.
The barium tetrafluoroberyllate is very insoluble and can be used for gravimetric analysis of beryllium.
H$_2$BeF$_4$ is an acid that can be produced from Ag$_2$BeF$_4$ and HCl. It only exists in aqueous solution.
Triglycine tetrafluoroberyllate (TGFB) is ferroelectric with a transition point of 70 °C. The crystals can be formed by dissolving BeF$_2$ in water, adding HF and then glycine. When the solution is cooled triglycine tetrafluoroberyllate forms. Cs$_2$BeF$_4$ and Tl$_2$BeF$_4$ 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 (NH$_4$)$_2$Mn(BeF$_4$)$_2$·6(H$_2$O) is made from a solution of NH$_4$BeF$_3$ mixed with NH$_4$MnF$_3$. 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 MgF$_2$.
| name | formula | molecular weight | crystal form | density |
| potassium lithium tetrafluoroberyllate | KLiBeF$_4$ | 131.05 | P6$_3$, a = 8.781 Å, b = 5.070 Å c = 8.566 Å | |
| rubidium lithium tetrafluoroberyllate | RbLiBeF$_4$ | 177.41 | P6$_3$22, a = 8.980 Å, b = 5.185 Å c = 8.751 Å | |
| caesium lithium tetrafluoroberyllate | CsLiBeF$_4$ | 224.852 | P2$_1$/n, a = 9.328 Å b = 5.356 Å, c = 8.736 Å, γ = 89.82° | |
| acid chromium fluoroberyllate tetracosihydrate | H$_2$Cr$_2$(BeF$_4$)$_4$·24H$_2$O | 878.40 |
| ammonium chromium fluoroberyllate tetracosihydrate | (NH$_4$)$_2$Cr$_2$(BeF$_4$)$_4$·24H$_2$O | 912.46 |
| rubidium chromium fluoroberyllate tetracosihydrate | Rb$_2$Cr$_2$(BeF$_4$)$_4$·24H$_2$O | 1047.32 |
| manganese ammonium fluoroberyllate hydrate | (NH$_4$)$_2$Mn(BeF$_4$)$_2$·6H$_2$O | 369.118 | | 1.758 |
| | Rb$_2$Fe(BeF$_4$)$_2$·6H$_2$O | 504.884 |
| ferrous ammonium fluoroberyllate hydrate | (NH$_4$)$_2$Fe(BeF$_4$)$_2$·6H$_2$O | 370.025 |
| nickel potassium fluoroberyllate hydrate | K$_2$Ni(BeF$_4$)$_2$·6H$_2$O | 414.913 |
| nickel rubidium fluoroberyllate hydrate | Rb$_2$Ni(BeF$_4$)$_2$·6H$_2$O | 507.732 |
| | Cs$_2$Ni(BeF$_4$)$_2$·6H$_2$O | 602.608 |
| nickel ammonium fluoroberyllate hydrate | (NH$_4$)$_2$Ni(BeF$_4$)$_2$·6H$_2$O | 372.874 | P2$_1$/a, a = 9.201 Å, b = 12.482 Å, c = 6.142 Å, β = 106.57 Å, V = 676.0 Å$^3$ Z = 2 | 1.843 |
| cobalt potassium fluoroberyllate hydrate | K$_2$Co(BeF$_4$)$_2$·6H$_2$O | 415.233 |
| cobalt rubidium fluoroberyllate hydrate | Rb$_2$Co(BeF$_4$)$_2$·6H$_2$O | 507.972 |
| cobalt ammonium fluoroberyllate hydrate | (NH$_4$)$_2$Co(BeF$_4$)$_2$·6H$_2$O | 372.874 | | 1.821 |
| copper rubidium fluoroberyllate hydrate | Rb$_2$Cu(BeF$_4$)$_2$·6H$_2$O | 512.585 |
| copper ammonium fluoroberyllate hydrate | (NH$_4$)$_2$Cu(BeF$_4$)$_2$·6H$_2$O | 377.726 | | 1.858 |
| zinc rubidium fluoroberyllate hydrate | Rb$_2$Zn(BeF$_4$)$_2$·6H$_2$O | 514.42 |
| zinc ammonium fluoroberyllate hydrate | (NH$_4$)$_2$Zn(BeF$_4$)$_2$·6H$_2$O | 379.56 | | 1.859 |
| cadmium rubidium fluoroberyllate hydrate | Rb$_2$Cd(BeF$_4$)$_2$·6H$_2$O | 561.45 |
| cadmium ammonium fluoroberyllate hydrate | (NH$_4$)$_2$Cd(BeF$_4$)$_2$·6H$_2$O | 426.591 |

### Alums

Tetrafluoroberyllate salts equivalent to alums also exist with formula MABF$_4$·12H$_2$O, 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.
| name | formula | crystal form | melting point |
| ammonium aluminium tetrafluoroberyllate alum | NH$_4$AlBeF$_4$·12H$_2$O | | |
| potassium aluminium tetrafluoroberyllate alum | KAlBeF$_4$·12H$_2$O | | |
| potassium chromium tetrafluoroberyllate alum | KCrBeF$_4$·12H$_2$O | | |
| ammonium chromium tetrafluoroberyllate alum | NH$_4$CrBeF$_4$·12H$_2$O | cubic a = 12.218 Å, Z = 4 | |
| rubidium chromium tetrafluoroberyllate alum | RbCrBeF$_4$·12H$_2$O | 12.214 Å | |
| caesium chromium tetrafluoroberyllate alum | CsCrBeF$_4$·12H$_2$O | 12.323 Å | |
| thallium chromium tetrafluoroberyllate alum | TlCrBeF$_4$·12H$_2$O | 12.195 Å | |
| rubidium iron tetrafluoroberyllate alum | RbFeBeF$_4$·12H$_2$O | | |
| caesium iron tetrafluoroberyllate alum | CsFeBeF$_4$·12H$_2$O | | |
| monomethyl chromium tetrafluoroberyllate alum | CH$_3$NH$_3$CrBeF$_4$·12H$_2$O | 12.496 Å | |
| guanidium chromium tetrafluoroberyllate alum | C(NH$_2$)$_3$CrBeF$_4$·12H$_2$O | 12.538 Å | on heating forms a rhombohedral hexahydrate stable from 30 °C to 90 °C |