Silicon tetrafluoride or tetrafluorosilane is a chemical compound with the formula SiF4. This colorless gas is notable for having a narrow liquid range: its boiling point is only 4 °C above its melting point. It was first prepared in 1771 by Carl Wilhelm Scheele by dissolving silica in hydrofluoric acid, and later synthesized by John Davy in 1812. It is a tetrahedral molecule and is corrosive.
Occurrence Volcanic plumes contain significant amounts of silicon tetrafluoride. Production can reach several tonnes per day. Some amounts are also emitted from spontaneous coal fires.Kruszewski, Ł., Fabiańska, M.J., Ciesielczuk, J., Segit, T., Orłowski, R., Motyliński, R., Moszumańska, I., Kusy, D. 2018 – First multi-tool exploration of a gas-condensate-pyrolysate system from the environment of burning coal mine heaps: An in situ FTIR and laboratory GC and PXRD study based on Upper Silesian materials. Science of the Total Environment, 640-641, 1044-1071; DOI: 10.1016/j.scitotenv.2018.05.319 The silicon tetrafluoride is partly hydrolysed and forms hexafluorosilicic acid.
Preparation is a by-product of the production of phosphate fertilizers wet process production, resulting from the attack of HF (derived from fluorapatite protonolysis) on silicates, which are present as impurities in the phosphate rocks. The hydrofluoric acid and silicon dioxide (SiO2) react to produce hexafluorosilicic acid: 6 HF + SiO2 → H2SiF6 + 2 H2O
In the laboratory, the compound is prepared by heating barium hexafluorosilicate (Ba[SiF6]) above whereupon the solid releases volatile , leaving a residue of .
Alternatively, sodium hexafluorosilicate () may also be thermally decomposed at — (optionally in inert nitrogen gas atmosphere)
Uses This volatile compound finds limited use in microelectronics and organic synthesis.Shimizu, M. "Silicon(IV) Fluoride" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley & Sons.
It is also used in production of fluorosilicic acid (see above).
In the 1980s, as part of the Low-Cost Solar Array Project by Jet Propulsion Laboratory, it was investigated as a potentially cheap feedstock for polycrystalline silicon production in fluidized bed reactors. Few methods using it for the said production process were patented.
The Ethyl Corporation process In the 1980s the Ethyl Corporation came up with a process that uses hexafluorosilicic acid and sodium aluminium hydride (NaAlH4) (or other alkali metal hydride) to produce silane (SiH4).
Safety In 2001 it was listed by New Jersey authorities as a hazardous substance that is corrosive and may severely irritate or even burn skin and eyes. It is fatal if inhaled.
See also
SiH4 (silane) Hexafluorosilicic acid
References