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|Section2= |Section3= }} 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.〔Wiberg, E., Wiberg, N., and Holleman, A. F. (2001) ''Inorganic chemistry''. Academic Press, San Diego.〕 It is made of white crystals,〔 which have at least three polymorphs〔 and appears in nature as rare minerals cryptohalite or bararite. ==Structure == 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.〔Klein, C. and Dutrow, B. (2008) ''The 23rd Edition of the Manual of Mineral Science''. John Wiley & Sons, Hoboken, NJ.〕) 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.〔To learn about the primitive hexagonal structure, see (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.〔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.〕 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.) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ammonium fluorosilicate」の詳細全文を読む スポンサード リンク
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