Silicon tetrafluoride or tetrafluorosilane is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one ele ...

with the formula Si F₄. 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 Carl Wilhelm Scheele (, ; 9 December 1742 – 21 May 1786) was a Swedish German pharmaceutical chemist. Scheele discovered oxygen (although Joseph Priestley published his findings first), and identified molybdenum, tungsten, barium, hydro ...

by dissolving silica in

hydrofluoric acid Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly corrosive. It is used to make most fluorine-containing compounds; examples include the commonly used pharmaceutical antidepr ...

., later synthesized by John Davy in 1812. It is a tetrahedral molecule and is corrosive.

Preparation

is a by-product of the production of

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

fertilizer A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...

s wet process production, resulting from the attack of HF (derived from

fluorapatite Fluorapatite, often with the alternate spelling of fluoroapatite, is a phosphate mineral with the formula Ca5(PO4)3F (calcium fluorophosphate). Fluorapatite is a hard crystalline solid. Although samples can have various color (green, brown, b ...

protonolysis) on silicates, which are present as impurities in the phosphate rocks. The

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 iF₆ above whereupon the solid releases volatile , leaving a residue of . : Alternatively,

sodium hexafluorosilicate Sodium fluorosilicate is a compound with the chemical formula Na2 iF6 Natural occurrence Sodium hexafluorosilicate occurs naturally as the rare mineral malladrite found within some volcanic fumaroles. Manufacturing Sodium fluorosilicate is made ...

() may also be thermally decomposed at — (optionally in inert nitrogen gas atmosphere) :

Uses

This volatile compound finds limited use in microelectronics and

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

. 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 The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA ...

, it was investigated as a potentially cheap feedstock for

polycrystalline silicon Polycrystalline silicon, or multicrystalline silicon, also called polysilicon, poly-Si, or mc-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry. Polysilicon is produ ...

production in fluidized bed reactors. Few methods using it for the said production process were patented.

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 Silane is an inorganic compound with chemical formula, . It is a colourless, pyrophoric, toxic gas with a sharp, repulsive smell, somewhat similar to that of acetic acid. Silane is of practical interest as a precursor to elemental silicon. Sila ...

(SiH₄).

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.

Safety

In 2001 it was listed by

New Jersey New Jersey is a U.S. state, state in the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. It is bordered on the north and east by the state of New York (state), New York; on the ea ...

authorities as a hazardous substance that is corrosive and may severely irritate or even burn skin and eyes. It is fatal if inhaled.

References