Ammonium hexafluorosilicate

Ammonium fluorosilicate

Names
IUPAC name Ammonium hexafluorosilicate
Other names Ammonium fluorosilicate Ammonium fluosilicate Ammonium silicofluoride
Identifiers
CAS Number	16919-19-0 Y
3D model (JSmol)	Interactive image Interactive image
ChemSpider	26182 Y
ECHA InfoCard	100.037.229
EC Number	240-968-3
PubChem CID	28145
RTECS number	VV7800000
UN number	2854
InChI InChI=1S/F6Si.2H3N/c1-7(2,3,4,5)6;;/h;2*1H3/q-2;;/p+2 Y Key: ITHIMUMYFVCXSL-UHFFFAOYSA-P Y InChI=1/F6Si.2H3N/c1-7(2,3,4,5)6;;/h;2*1H3/q-2;;/p+2 Key: ITHIMUMYFVCXSL-SKRXCDHZAK
SMILES F[Si-2](F)(F)(F)(F)F.[NH4+].[NH4+] [NH4+].[NH4+].F[Si--](F)(F)(F)(F)F
Properties
Chemical formula	(NH4)2[SiF6]
Appearance	White crystals
Density	2.0 g cm−3
Melting point	100 °C (212 °F; 373 K) (decomposes)
Solubility in water	dissolves in water and alcohol
Related compounds
Other cations	Hexafluorosilicic acid Sodium fluorosilicate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
Infobox references

Ammonium fluosilicate

Sodium fluorosilicate

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)₂[SiF₆] 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 [111] 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)₂[SiF₆] (alpha), have cubic close(st) packing (CCP). The γ form has hexagonal close(st) packing and the β-(NH₄)₂[SiF₆] 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.