{{Short description|Chemical compound}} {{Chembox | Watchedfields = changed | verifiedrevid = 455356187 | ImageFileL1 = Diammonium-hexafluorosilicate-unit-cell-3D-balls-A.png | ImageClassL1 = bg-transparent | ImageAltL1 = Ball-and-stick model for a unit cell. | ImageFileR1 = Bararite & Cryptohalite.jpg | ImageAltR1 = Two crystals with small light yellow-orange ice-like grains twining side-by-side, but a clear gap between; selenium contamination darkens the left one. | IUPACName = Ammonium hexafluorosilicate | SystematicName = <!-- Diazanium hexafluorosilanediuide (substitutive) OR Diazanium hexafluoridosilicate(2−) (additive) --> <!-- Please, do not change the official IUPAC name without consensus --> | OtherNames = Ammonium fluorosilicate<br /> Ammonium fluosilicate<br /> Ammonium silicofluoride |Section1={{Chembox Identifiers | CASNo = 16919-19-0 | CASNo_Ref = {{cascite|correct|CAS}} | UNII_Ref = {{fdacite|correct|FDA}} | UNII = 1X545W620G | PubChem = 28145 | ChemSpiderID = 26182 | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | EINECS = 240-968-3 | UNNumber = 2854 | RTECS = VV7800000 | SMILES = F[Si-2](F)(F)(F)(F)F.[NH4+].[NH4+] | SMILES1 = [NH4+].[NH4+].F[Si--](F)(F)(F)(F)F | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChI = 1S/F6Si.2H3N/c1-7(2,3,4,5)6;;/h;2*1H3/q-2;;/p+2 | InChI = 1/F6Si.2H3N/c1-7(2,3,4,5)6;;/h;2*1H3/q-2;;/p+2 | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = ITHIMUMYFVCXSL-UHFFFAOYSA-P | InChIKey = ITHIMUMYFVCXSL-SKRXCDHZAK}} |Section2={{Chembox Properties | Formula = (NH<sub>4</sub>)<sub>2</sub>[SiF<sub>6</sub>] | Appearance = White crystals | Density = 2.0 g cm<sup>−3</sup> | MeltingPtC = 100 | MeltingPt_notes = (decomposes)<ref>[http://chemicalland21.com/industrialchem/inorganic/AMMONIUM%20SILICOFLUORIDE.htm ammonium silicofluoride]</ref> | SolubleOther = dissolves in water and alcohol}} |Section7={{Chembox Hazards | Hazards_ref = <ref>{{cite web|url=https://pubchem.ncbi.nlm.nih.gov/compound/Ammonium_hexafluorosilicate|title=Ammonium Hexafluorosilicate|website=PubChem|publisher=National Institute of Health|access-date=October 12, 2017}}</ref><ref>{{cite web|url=https://www.fishersci.com/store/msds?partNumber=AC310930250&productDescription=AMMONIUM+HEXAFLUOROSILIC+25GR&vendorId=VN00032119&countryCode=US&language=en|title=SAFETY DATA SHEET Ammonium hexafluorosilicate, 99.999%|website=Fisher Scientific|archive-url=https://web.archive.org/web/20171016122519/https://www.fishersci.com/store/msds?partNumber=AC310930250&productDescription=AMMONIUM+HEXAFLUOROSILIC+25GR&vendorId=VN00032119&countryCode=US&language=en|archive-date=2017-10-16|url-status=dead|access-date=October 15, 2017}}</ref> | ExternalSDS = [https://www.chemicalbook.com/ProductMSDSDetailCB0691538_EN.htm ChemicalBook MSDS] | NFPA-H = 3 | NFPA-F = 0 | NFPA-R = 0 | NFPA-S = ACID | GHSPictograms = {{GHS05|Corrosive}} {{GHS06|Acute Toxicity}} {{GHS08|target organ toxicity}} | GHSSignalWord = DANGER | HPhrases = {{H-phrases|301|311|315|319|331|335|372}} | PPhrases = {{P-phrases|260|261|264|270|271|280|301+310|302+352|304+340|305+351+338|312|314|321|330|332+313|337+313|362|403+233|405|501}} }} |Section3={{Chembox Related | OtherCations = Hexafluorosilicic acid<br /> Sodium fluorosilicate}} }} '''Ammonium fluorosilicate''' (also known as ammonium hexafluorosilicate, ammonium fluosilicate or ammonium silicofluoride) has the formula (NH<sub>4</sub>)<sub>2</sub>SiF<sub>6</sub>. It is a toxic chemical, like all salts of fluorosilicic acid.<ref name="Wiberg">Wiberg, E., Wiberg, N., and Holleman, A. F. (2001) ''Inorganic chemistry''. Academic Press, San Diego.</ref> It is made of white crystals,<ref name="Cameo"/> which have at least three polymorphs<ref name="Boldyreva"/> and appears in nature as rare minerals cryptohalite or bararite.
==Structure == Ammonium fluorosilicate has three major polymorphs: α-(NH<sub>4</sub>)<sub>2</sub>[SiF<sub>6</sub>] form is cubic (space group Fm<u style="text-decoration:overline">3</u>m, No. 225) and corresponds to the mineral cryptohalite. The β form is trigonal (scalenohedral) and occurs in nature as mineral bararite.<ref name=anth/> A third (γ) form was discovered in 2001 and identified with the hexagonal 6mm symmetry. In all three configurations, the [SiF<sub>6</sub>]<sup>2−</sup> octahedra are arranged in layers. In the α form, these layers are perpendicular to [[Miller index|[111]]] directions. In the β- and γ- forms, the layers are perpendicular to the c-axis.<ref name="Boldyreva">{{cite journal|author =Boldyreva, E. V.|author-link1=Elena Boldyreva |author2=Shakhtshneider, T. P. |author3=Sowa, H. |author4=Ahsbas, H. |year=2007|title=Effect of hydrostatic pressure up to 6 GPa on the crystal structures of ammonium and sodium hexafluorosilicates, (NH<sub>4</sub>)<sub>2</sub>[SiF<sub>6</sub>] and Na<sub>2</sub>[SiF<sub>6</sub>]; a phase transition in (NH<sub>4</sub>)<sub>2</sub>[SiF<sub>6</sub>] at 0.2–0.3 GPa|journal=Zeitschrift für Kristallographie|pages=23–29|doi=10.1524/zkri.2007.222.1.23|volume=222|s2cid=97174719 }}</ref> (Note: trigonal symmetry is part of the hexagonal group, but not all hexagonal crystals are trigonal.<ref name="Klein">Klein, C. and Dutrow, B. (2008) ''The 23rd Edition of the Manual of Mineral Science''. John Wiley & Sons, Hoboken, NJ.</ref>) The silicon atoms of α-(NH<sub>4</sub>)<sub>2</sub>[SiF<sub>6</sub>] (alpha), have cubic close packing (CCP). The γ form has hexagonal close packing and the β-(NH<sub>4</sub>)<sub>2</sub>[SiF<sub>6</sub>] has primitive hexagonal packing.<ref>To learn about the primitive hexagonal structure, see [http://www.polymorf.net/matter4.htm Primitive hexagonal packing] {{webarchive|url=https://web.archive.org/web/20090526142134/http://www.polymorf.net/matter4.htm |date=2009-05-26 }}.</ref> In all three phases, 12 fluorine atoms neighbor the (NH<sub>4</sub>)<sup>+</sup>.<ref name="Boldyreva"/>
Although bararite was claimed to be metastable at room temperature,<ref name="Schlemper2">{{cite journal|doi=10.1063/1.1727071|title=Structure of Cubic Ammonium Fluosilicate: Neutron-Diffraction and Neutron-Inelastic-Scattering Studies|year=1966|last1=Schlemper|first1=Elmer O.|journal=The Journal of Chemical Physics|volume=44|pages=2499–2505|issue=6|bibcode=1966JChPh..44.2499S }}</ref> it does not appear one polymorph has ever turned into another.<ref name="Boldyreva"/> Still, bararite is fragile enough that grinding it for spectroscopy will produce a little cryptohalite.<ref name="Oxton">Oxton, I. A., Knop, O., and Falk, M. (1975) [https://archive.today/20121216022602/http://article.pubs.nrc-cnrc.gc.ca/ppv/RPViewDoc?issn=1480-3291&volume=53&issue=22&startPage=3394 "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.</ref> 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.<ref name="Boldyreva"/>
The hydrogen bonding in (NH<sub>4</sub>)<sub>2</sub>[SiF<sub>6</sub>] 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.<ref name="Boldyreva"/> (To learn more about the β-structure, see Bararite.)
==Natural occurrence== This chemical makes rare appearances in nature.<ref>Barnes, J. and Lapham, D. (1971) "Rare Minerals Found in Pennsylvania". ''Pennsylvania Geology'', 2, 5, 6–8.</ref> It is found as a sublimation product of fumaroles and coal fires. As a mineral, it is either called cryptohalite or bararite, the two being two polymorphs of the compound.<ref name=anth>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. *[http://www.handbookofmineralogy.org/pdfs/BARARITE.pdf link to bararite] {{Webarchive|url=https://web.archive.org/web/20160401193238/http://www.handbookofmineralogy.org/pdfs/Bararite.pdf |date=2016-04-01 }} *[http://www.handbookofmineralogy.org/pdfs/CRYPTOHALITE.pdf link to cryptohalite] {{Webarchive|url=https://web.archive.org/web/20211202010021/http://www.handbookofmineralogy.org/pdfs/Cryptohalite.pdf |date=2021-12-02 }}</ref>
==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 an acid solution.<ref name="Cameo"/>
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.<ref name="Cameo"/>
==Uses== Ammonium fluorosilicate finds use as a disinfectant, and it is useful in etching glass, metal casting, and electroplating.<ref name="Cameo">[http://cameochemicals.noaa.gov/chemical/5407 Ammonium fuorosilicate], CAMEO Chemicals, NOAA</ref> It is also used to help neutralize washing machine water as laundry sour.
==See also== {{commons category|Diammonium hexafluorosilicate}} * Hexafluorosilicic acid * Sodium fluorosilicate
==References== {{reflist|30em}}
== External links == *[https://web.archive.org/web/20090526142134/http://www.polymorf.net/matter4.htm Primitive hexagonal packing]
<!-- Please, do not overcategorize, nor apply not agreed chemical categories -->
{{fluorine compounds}} {{Ammonium salts}} {{Authority control}}
Category:Ammonium compounds Category:Hexafluorosilicates