The Structure of Ammonium Fluorosilicate

Ammonium fluorosilicate which is 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 [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)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.

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