Silicon tetrafluoride explained

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., later synthesized by John Davy in 1812.^[1] It is a tetrahedral molecule and is corrosive.^[2]

Occurrence

Volcanic plumes contain significant amounts of silicon tetrafluoride. Production can reach several tonnes per day.^[3] Some amounts are also emitted from spontaneous coal fires.^[4] 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.^[5] 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 300C whereupon the solid releases volatile, leaving a residue of .

Alternatively, sodium hexafluorosilicate may also be thermally decomposed at 400C—600C (optionally in inert nitrogen gas atmosphere) ^[6]

Uses

This volatile compound finds limited use in microelectronics and organic synthesis.^[7]

It's also used in production of fluorosilicic acid (see above).

Staying in the 1980s, as part of the Low-Cost Solar Array Project by Jet Propulsion Laboratory,^[8] it was investigated as a potentially cheap feedstock for polycrystalline silicon production in fluidized bed reactors.^[9] Few methods using it for the said production process were patented.^[10]

The Ethyl Corporation process

In 80s 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).^[11]

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.^[12]

See also

• SiH₄ (silane)
• Hexafluorosilicic acid

Notes and References

1. An Account of Some Experiments on Different Combinations of Fluoric Acid. John Davy. Philosophical Transactions of the Royal Society of London. 1812. 102. 352–369. 10.1098/rstl.1812.0020. 0261-0523. 107324. free.
2. Web site: November 2001 . Hazardous Substance Fact Sheet . New Jersey Department of Health and Senior services.
3. High SiF4/HF ratio detected in Satsuma-Iwojima volcano's plume by remote FT-IR observation. T. Mori . M. Sato . Y. Shimoike . K. Notsu . Earth Planets Space. 2002 . 54. 3. 249–256. 10.1186/BF03353024 . 2002EP&S...54..249M . 55173591 . free.
4. 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
5. Book: Inorganic Syntheses . 1953 . 4 . 147–8 . 10.1002/9780470132357.ch48 . Hoffman, C. J. . Gutowsky, H. S. . Germanium(IV) Fluoride . 978-0-470-13163-3 .
6. Us. A345458. Patent Silicon tetrafluoride generation. Granted. January 3, 1982. 1982. Keith. L. Yaws. C. Hansen. Carl.
7. Shimizu, M. "Silicon(IV) Fluoride" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley & Sons.
8. Book: Callaghan, William T. . Photovoltaic Solar Energy Conference . Low-Cost Solar Array Project Progress and Plans . 1981 . Palz . W. . https://link.springer.com/chapter/10.1007/978-94-009-8423-3_40 . en . Dordrecht . Springer Netherlands . 279–286 . 10.1007/978-94-009-8423-3_40 . 978-94-009-8423-3.
9. Acharya . H. N. . Datta . S. K. . Banerjee . H. D. . Basu . S. . 1982-09-01 . Low-temperature preparation of polycrystalline silicon from silicon tetrachloride . Materials Letters . en . 1 . 2 . 64–66 . 10.1016/0167-577X(82)90008-8 . 0167-577X.
10. CA. 2741023A1. Method for the production of polycrystalline silicon. 2008. Anatoli. Tozzoli. V. Pushko. Silvio.
11. Web site: August 11, 2015 . The Ethyl Corporation Process: Silane and Fluidised Bed Reactor .
12. Web site: April 9, 2018 . SAFETY DATA SHEET: Silicon Tetrafluoride . Airgas.