Sulfur hexafluoride or sulphur hexafluoride (British spelling) is an inorganic compound with the formula SF6. It is a colorless, odorless, non-flammable, and non-toxic gas. SF6 has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, SF6 is poorly soluble in water but quite soluble in nonpolar organic solvents. It has a density of 6.12 g/L at sea level conditions, considerably higher than the density of air (1.225 g/L). It is generally stored and transported as a liquefied compressed gas. SF6 has 23,500 times greater global warming potential (GWP) than CO2 as a greenhouse gas (over a 100-year time horizon), but exists in relatively minor concentrations in the atmosphere. Its GWP is 35,000 on a 500-year time horizon, compared with CO2. Its concentration in Earth's troposphere reached 12.06 parts per trillion (ppt) in February 2025, rising at 0.4 ppt/year. The increase since 1980 is driven in large part by the expanding electric power sector, including fugitive emissions from banks of SF6 gas contained in its medium- and high-voltage switchgear. Uses in magnesium, aluminium, and electronics manufacturing also hastened atmospheric growth.
Synthesis and reactions
Sulfur hexafluoride on Earth exists primarily as a synthetic industrial gas, but has also been found to occur naturally. SF6 can be prepared from the elements through exposure of S8 to F2. This was the method used by the discoverers Henri Moissan and Paul Lebeau in 1901. Some other sulfur fluorides are cogenerated, but these are removed by heating the mixture to disproportionate any S2F10 (which is highly toxic) and then scrubbing the product with NaOH to destroy the remaining SF4. Alternatively, using bromine, sulfur hexafluoride can be synthesized from SF4 and CoF3 at lower temperatures (e.g. 100 °C), as follows:
There are few chemical reactions for SF6. A main contribution to the inertness of SF6 is the steric hindrance of the sulfur atom, whereas its heavier group 16 counterparts, such as SeF6 are more reactive than SF6 as a result of less steric hindrance. It does not react with molten sodium below its boiling point, but reacts exothermically with lithium. However, alkali metals react with SF6 in liquid ammonia to form the corresponding sulfides and fluorides:
As a result of its inertness, SF6 has an atmospheric lifetime of around 3200 years, and no significant environmental sinks other than the ocean.
Applications By 2000, the electrical power industry is estimated to use about 80% of the sulfur hexafluoride produced, mostly as a gaseous dielectric medium. Other main uses as of 2015 included a silicon etchant for semiconductor manufacturing, and an inert gas for the casting of magnesium.
Dielectric medium SF6 is used in the electrical industry as a gaseous dielectric medium for high-voltage sulfur hexafluoride circuit breakers, gas insulated switchgear (GIS), and other electrical equipment, often replacing oil-filled circuit breakers (OCBs) that can contain harmful polychlorinated biphenyls (PCBs). SF6 gas under pressure is used as an insulator in GIS because it has a much higher dielectric strength than air or dry nitrogen. The high dielectric strength is a result of the gas's high electronegativity and density. This property makes it possible to significantly reduce the size of electrical gear. This makes GIS more suitable for certain purposes such as indoor placement, as opposed to air-insulated electrical gear, which takes up considerably more room. Gas-insulated electrical gear is also more resistant to the effects of pollution and climate, as well as being more reliable in long-term operation because of its controlled operating environment. Exposure to an arc chemically breaks down SF6 though most of the decomposition products tend to quickly re-form SF6, a process termed "self-healing". Arcing or corona can produce disulfur decafluoride (S2F10), a highly toxic gas, with toxicity similar to phosgene. S2F10 was considered a potential chemical warfare agent in World War II because it does not produce lacrimation or skin irritation, thus providing little warning of exposure. SF6 is also commonly encountered as a high voltage dielectric in the high voltage supplies of particle accelerators, such as Van de Graaff generators and Pelletrons and high voltage transmission electron microscopes. Alternatives to SF6 as a dielectric gas include several fluoroketones. Compact GIS technology that combines vacuum switching with clean air insulation has been introduced for a subset of applications up to 420 kV.
Medical use SF6 is used to provide a tamponade or plug of a retinal hole in retinal detachment repair operations in the form of a gas bubble. It is inert in the vitreous chamber. The bubble initially doubles its volume in 36 hours due to oxygen and nitrogen entering it, before being absorbed in the blood in 10–14 days. SF6 is used as a contrast agent for ultrasound imaging. Sulfur hexafluoride microbubbles are administered in solution through injection into a peripheral vein. These microbubbles enhance the visibility of blood vessels to ultrasound. This application has been used to examine the vascularity of tumours. It remains visible in the blood for 3 to 8 minutes, and is exhaled by the lungs.
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