Silicon tetrafluoride

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.

Volcanic plumes contain significant amounts of silicon tetrafluoride. Production can reach several tonnes per day. Some amounts are also emitted from spontaneous coal fires. The silicon tetrafluoride is partly hydrolysed and forms hexafluorosilicic acid.

SiF
₄ 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 300 °C (572 °F) whereupon the solid releases volatile SiF
₄, leaving a residue of BaF
₂.

Ba[SiF₆] + 400°C → BaF₂ + SiF₄

Alternatively, sodium hexafluorosilicate (Na₂[SiF₆]) may also be thermally decomposed at 400 °C (752 °F)—600 °C (1,112 °F) (optionally in inert nitrogen gas atmosphere)

Na₂[SiF₆] + 400°C → 2NaF + SiF₄

This volatile compound finds limited use in microelectronics and organic synthesis.

It is 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, 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.

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).

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.

• SiH₄ (silane)
• Hexafluorosilicic acid

• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.