|Jmol-3D images||Image 1
|Molar mass||178.15 g mol−1|
|Density||2.0 g cm−3|
|Melting point||100 °C; 212 °F; 373 K (decomposes)|
|Solubility in water||dissolves in water and alcohol|
|Other cations||Hexafluorosilicic acid
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Ammonium fluorosilicate (also known as ammonium hexafluorosilicate, ammonium fluosilicate or ammonium silicofluoride) has the formula (NH4)2SiF6. It is a toxic chemical, like all salts of fluorosilicic acid. It is made of white crystals, which have at least three polymorphs and appears in nature as rare minerals cryptohalite or bararite.
Ammonium fluorosilicate has three major polymorphs: α-(NH4)2SiF6 form is cubic (space group Fm3m, No. 225) and corresponds to the mineral cryptohalite. The β form is trigonal (scalenohedral) and occurs in nature as mineral bararite. A third (γ) form was discovered in 2001 and identified with the hexagonal 6mm symmetry. In all three configurations, the (SiF6)2− octahedra are arranged in layers. In the α form, these layers are perpendicular to  directions. In the β- and γ- forms, the layers are perpendicular to the c-axis. (Note: trigonal symmetry is part of the hexagonal group, but not all hexagonal crystals are trigonal.) The silicon atoms of α-(NH4)2SiF6 (alpha), have cubic close(st) packing (CCP). The γ form has hexagonal close(st) packing and the β-(NH4)2SiF6 has primitive hexagonal packing. In all three phases, 12 fluorine atoms neighbor the (NH4)+.
Although bararite was claimed to be metastable at room temperature, it does not appear one polymorph has ever turned into another. Still, bararite is fragile enough that grinding it for spectroscopy will produce a little cryptohalite. Even so, ammonium fluorosilicate assumes a trigonal form at pressures of 0.2 to 0.3 GPa. The reaction is irreversible. If it is not bararite, the phase is at least very closely related.
The hydrogen bonding in (NH4)2SiF6 allows this salt to change phases in ways that normal salts cannot. Interactions between cations and anions are especially important in how ammonium salts change phase. (To learn more about the β-structure, see Bararite.)
Chemical properties and health hazards
Ammonium fluorosilicate is noncombustible, but it will still release dangerous fumes in a fire, including hydrogen fluoride, silicon tetrafluoride, and nitrogen oxides. It will corrode aluminium. In water, ammonium fluorosilicate dissolves to form a basic solution.
Inhaling dust can lead to pulmonary irritation, possibly death. Ingestion may also prove fatal. Irritation of the eyes comes from contact with the dust, as well as irritation or ulceration of the skin.
Ammonium fluorosilicate finds use as a disinfectant, and it is useful in etching glass, metal casting, and electroplating.
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- ammonium silicofluoride
- Wiberg, E., Wiberg, N., and Holleman, A. F. (2001) Inorganic chemistry. Academic Press, San Diego.
- Ammonium fuorosilicate, CAMEO Chemicals, NOAA
- Boldyreva, E. V., Shakhtshneider, T. P., Sowa, H., and Ahsbas, H. (2007). "Effect of hydrostatic pressure up to 6 GPa on the crystal structures of ammonium and sodium hexafluorosilicates, (NH4)2SiF6 and Na2SiF6; a phase transition in (NH4)2SiF6 at 0.2–0.3 GPa". Zeitschrift für Kristallographie 222: 23–29. doi:10.1524/zkri.2007.222.1.23.
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- To learn about the primitive hexagonal structure, see Primitive hexagonal packing.
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