Magnesium fluoride

Magnesium fluoride[1]
Other names	Sellaite Irtran-1
Identifiers
CAS number	7783-40-6 Y
PubChem	24546
RTECS number	OM3325000
Properties
Molecular formula	MgF2
Molar mass	62.3018 g/mol
Appearance	white tetragonal crystals
Density	3.148 g/cm3
Melting point	1261°C [2]
Boiling point	2260°C (decomp)
Solubility in water	0.013 g/100 mL
Solubility product, Ksp	5.16·10-11
Solubility	soluble in nitric acid insoluble in ethanol
Refractive index (nD)	1.37397
Structure
Crystal structure	Rutile (tetragonal), tP6
Space group	P42/mnm, No. 136
Thermochemistry
Std enthalpy of formation ΔfHo298	-1124.2 kJ·mol-1
Standard molar entropy So298	57.2 J·mol-1·K-1
Specific heat capacity, C	61.6 J·mol-1·K-1
Related compounds
Other anions	Magnesium chloride Magnesium bromide Magnesium iodide
Other cations	Calcium 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

Magnesium fluoride is an inorganic compound with the formula MgF₂. This white crystalline salt is useful because it is transparent over a wide range of energies.

Production and structure

Magnesium fluoride is prepared from magnesium oxide with sources of HF such as ammonium bifluoride:

MgO + (NH₄)HF₂ → MgF₂ + NH₃ + H₂O

The compound crystallizes as tetragonal birefringent crystals. Its structure is similar to that in rutile, featuring octahedral Mg²⁺ centers and 3-coordinate fluoride.^[3]

Uses

Optical properties

Magnesium fluoride is transparent over an extremely wide range of wavelengths. Windows, lenses, and prisms made of this material can be used over the entire range of wavelengths from 0.120 μm (vacuum ultraviolet) to 8.0 μm (infrared). High quality synthetic VUV grade MgF₂ is quite expensive, in the region of $3000/kg (2007) but the real cost of optics in this material is due to relatively low volume manufacture. However, with lithium fluoride it is one of the two materials that will transmit in the vacuum ultraviolet range at 121 nm (Lyman alpha) and this is where it finds its application. Lower grade MgF₂ is sometimes used in the infrared but here it is inferior to calcium fluoride. MgF₂ is tough and works and polishes well, but it is slightly birefringent and should be cut with the optic axis perpendicular to the plane of the window or lens.

The Verdet constant of (MgF₂) at 632.8 nm is 0.00810 arcmin G^-1 cm^-1[4]

Thin layers of MgF₂ are frequently applied to the surfaces of optical elements as part of optical coatings such as anti-reflective coatings.

Other

MgF₂ is also used as a flux.

References

External links

• A java applet showing the effect of MgF₂ on a lens
• Infrared windows at Lawrence Berkeley National Laboratory
• National Pollutant Inventory - Fluoride and compounds fact sheet

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