Ammonium fluorosilicate

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Ammonium fluorosilicate
Diammonium-hexafluorosilicate-unit-cell-3D-balls-A.png
Names
IUPAC name
Ammonium hexafluorosilicate
Other names
Ammonium fluorosilicate

Ammonium fluosilicate

Ammonium silicofluoride
Identifiers
16919-19-0 YesY
ChemSpider 26182 YesY
EC number 240-968-3
Jmol-3D images Image
Image
PubChem 28145
RTECS number VV7800000
UN number 2854
Properties
H8F6N2Si
Molar mass 178.15 g·mol−1
Appearance White crystals
Density 2.0 g cm−3
Melting point 100 °C (212 °F; 373 K) (decomposes)[1]
dissolves in water and alcohol
Related compounds
Other cations
Hexafluorosilicic acid

Sodium fluorosilicate

Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

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.[2] It is made of white crystals,[3] which have at least three polymorphs[4] and appears in nature as rare minerals cryptohalite or bararite.

Structure[edit]

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.[5] 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 [111] directions. In the β- and γ- forms, the layers are perpendicular to the c-axis.[4] (Note: trigonal symmetry is part of the hexagonal group, but not all hexagonal crystals are trigonal.[6]) 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.[7] In all three phases, 12 fluorine atoms neighbor the (NH4)+.[4]

Although bararite was claimed to be metastable at room temperature,[8] it does not appear one polymorph has ever turned into another.[4] Still, bararite is fragile enough that grinding it for spectroscopy will produce a little cryptohalite.[9] 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.[4]

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.[4] (To learn more about the β-structure, see Bararite.)

Natural occurrence[edit]

This chemical makes rare appearances in nature.[10] It is found as a sublimation product of fumaroles and coal fires. As a mineral, it is called cryptohalite or bararite.[5]

Chemical properties and health hazards[edit]

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 an acid solution.[3]

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.[3]

Uses[edit]

Ammonium fluorosilicate finds use as a disinfectant, and it is useful in etching glass, metal casting, and electroplating.[3]

See also[edit]

References[edit]

  1. ^ ammonium silicofluoride
  2. ^ Wiberg, E., Wiberg, N., and Holleman, A. F. (2001) Inorganic chemistry. Academic Press, San Diego.
  3. ^ a b c d Ammonium fuorosilicate, CAMEO Chemicals, NOAA
  4. ^ a b c d e f 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. 
  5. ^ a b Anthony, J. W., Bideaux, R. A., Bladh, K. W., and Nichols, M. C. (1997) Handbook of Mineralogy, Volume III: Halides, Hydroxides, Oxides. Mineral Data Publishing, Tucson.
  6. ^ Klein, C. and Dutrow, B. (2008) The 23rd Edition of the Manual of Mineral Science. John Wiley & Sons, Hoboken, NJ.
  7. ^ To learn about the primitive hexagonal structure, see Primitive hexagonal packing.
  8. ^ Schlemper, Elmer O. (1966). "Structure of Cubic Ammonium Fluosilicate: Neutron-Diffraction and Neutron-Inelastic-Scattering Studies". The Journal of Chemical Physics 44 (6): 2499. doi:10.1063/1.1727071. 
  9. ^ Oxton, I. A., Knop, O., and Falk, M. (1975) "Infrared Spectra of the Ammonium Ion in Crystals". II. The Ammonium Ion in Trigonal Environments, with a Consideration of Hydrogen Bonding. Canadian Journal of Chemistry, 53, 3394–3400.
  10. ^ Barnes, J. and Lapham, D. (1971) "Rare Minerals Found in Pennsylvania". Pennsylvania Geology, 2, 5, 6–8.

External links[edit]