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,[5] and later synthesized by John Davy in 1812.[6] It is a tetrahedral molecule and is corrosive.[7]
Occurrence
Volcanic plumes contain significant amounts of silicon tetrafluoride. Production can reach several tonnes per day.[8] Some amounts are also emitted from spontaneous coal fires.[9] The silicon tetrafluoride is partly hydrolysed and forms hexafluorosilicic acid.
Preparation
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.[10] The hydrofluoric acid and silicon dioxide (SiO2) react to produce hexafluorosilicic acid:[10]
- 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.