Calcium fluoride

Overview

Calcium fluoride (CaF₂) is an insoluble ionic compound of calcium and fluorine. It occurs naturally as the mineral fluorite (also called fluorspar), and it is the source of most of the world's fluorine. This insoluble solid adopts a cubic structure wherein calcium is coordinated to eight fluoride anions and each F^- ion is surrounded by four Ca²⁺ ions. Although the pure material is colourless, the mineral is often deeply coloured due to the presence of F-centers.

Source of HF

Naturally occurring CaF₂ is the principal source of hydrogen fluoride, a commodity chemical used to produce a wide range of materials. Fluoride is liberated from the mineral by the action of concentrated sulfuric acid:

CaF₂(s) + H₂SO₄(l) → CaSO₄(solid) + 2 HF(g)

The resulting HF is converted into fluorine, fluorocarbons, and diverse fluoride materials. As of the late 1990s, five billion kilograms were mined annually.

Other applications

Calcium fluoride is commonly used as a window material for both infrared and ultraviolet wavelengths, since it is transparent in these regions (about 0.15 µm to 9 µm) and exhibits extremely weak birefringence. Furthermore the material is fairly inert chemically so that these windows are not attacked. Nevertheless, at wavelengths as low as 157 nm, which are interesting to semiconductor manufacturers, the birefringence of calcium fluoride exceeds tolerable limits. This problem with birefringence can be mitigated through optimised growth process. It is particularly important as an ultraviolet optical material for integrated circuit lithography. Canon also uses artificially-crystallized calcium fluoride components in some of its L-series lenses to reduce light dispersion. As an infrared optical material, calcium fluoride is sometimes known by the Eastman Kodak trademarked name "Irtran-3," although this designation is obsolete.

Uranium-doped calcium fluoride was the second type of solid state laser invented, in the 1960s. Peter Sorokin and Mirek Stevenson at IBM's laboratories in Yorktown Heights (US) achieved lasing at 2.5 µm shortly after Maiman's ruby laser.

It is also used as a flux for melting and liquid processing of iron, steel and their composites. Its action is based on its similar melting point to iron, on its ability to dissolve oxides and on its ability to wet oxides and metals.

Safety

Fluorides are toxic to humans, however CaF₂ is considered relatively harmless due to its extreme insolubility. The situation is analogous to BaSO₄, where the toxicity normally associated with Ba²⁺ is offset by the very low solubility of its sulfate derivative.

See also

• List of laser types
• Photolithography

Related materials

• Barium fluoride
• Magnesium fluoride
• Calcium chloride

External links

• NIST webbook thermochemistry data
• Uranium doped CaF₂ laser (pdf file)
• Charles Townes on the history of lasers
• National Pollutant Inventory - Flouride and compounds fact sheet
• MSDS (University of Oxford)