calcium fluoride

Calcium fluoride
Identifiers
CAS number	7789-75-5 Y
PubChem	24617
EC number	232-188-7
RTECS number	EW1760000
SMILES	[Ca+2].[F-].[F-]
InChI	1/Ca.2FH/h;2*1H/q+2;;/p-2
InChI key	WUKWITHWXAAZEY-NUQVWONBAZ
ChemSpider ID	23019
Properties
Molecular formula	CaF2
Molar mass	78.07 g mol−1
Appearance	White crystalline solid (single crystals are transparent)
Density	3.18 g/cm3
Melting point	1418 °C, 1691 K, 2584 °F
Boiling point	2533 °C, 2806 K, 4591 °F
Solubility in water	0.0015 g/100 mL (18 °C) 0.0016 g/100 mL (20 °C)
Solubility product, Ksp	3.9 x 10-11 [1]
Solubility in acetone	insoluble
Refractive index (nD)	1.4328
Structure
Crystal structure	cubic crystal system, cF12[2]
Space group	Fm3m, #225
Coordination geometry	Ca, 8, cubic F, 4, tetrahedral
Hazards
EU Index	Not listed
Main hazards	Reacts with conc. sulfuric acid to produce hydrofluoric acid
NFPA 704	0 3 0
Flash point	Non-flammable
LD50	4250 mg/kg (oral, rat)
Related compounds
Other anions	Calcium chloride Calcium bromide Calcium iodide
Other cations	Magnesium fluoride Strontium fluoride Barium fluoride
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Calcium fluoride (CaF₂) is an insoluble ionic compound of calcium and fluorine. It occurs naturally as the mineral fluorite (also called fluorspar) and as Blue John, 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.^[3] 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.^[4]

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 low refractive index. Furthermore the material is fairly inert chemically so that these windows are not attacked. Nevertheless, at wavelengths as low as 157 nm, which are useful for semiconductor manufacturers, the refractive index of calcium fluoride exceeds tolerable limits. This problem 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.

References

1. ^ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0070494398
2. ^ X-ray Diffraction Investigations of CaF₂ at High Pressure, L. Gerward, J. S. Olsen, S. Steenstrup, M. Malinowski, S. Åsbrink and A. Waskowska, Journal of Applied Crystallography (1992), 25, 578-581 doi:10.1107/S0021889892004096
3. ^ G. L. Miessler and D. A. Tarr “Inorganic Chemistry” 3rd Ed, Pearson/Prentice Hall publisher, ISBN 0-13-035471-6.
4. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.

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 - Fluoride and compounds fact sheet
• MSDS (University of Oxford)