Magnesium fluoride
Template:Short description Template:MCN Template:Chembox Magnesium fluoride is an ionically bonded inorganic compound with the formula Template:Chem2. The compound is a colorless to white crystalline salt that is transparent over a wide range of wavelengths, such that it is used in the optical windows of space telescopes. It occurs naturally as the rare mineral sellaite.
Production
Magnesium fluoride is prepared from magnesium oxide with sources of hydrogen fluoride such as ammonium bifluoride, by the breakdown of it:
Related metathesis reactions are also feasible:
Structure
The compound crystallizes as tetragonal birefringent crystals. The structure of the magnesium fluoride is similar to that of rutile,<ref name="Wells">Template:Cite book</ref><ref name="G&E">Template:Greenwood&Earnshaw2nd</ref> featuring octahedral Template:Chem2 cations and 3-coordinate [[fluoride|Template:Chem2]] anions.<ref name=Aigueperse>Template:Ullmann</ref>
| Magnesium coordination | Fluorine coordination |
|---|---|
| File:Magnesium-fluoride-xtal-Mg-coord-3D-bs-17.png | File:Magnesium-fluoride-xtal-F-coord-3D-bs-17.png |
In the gas phase, monomeric Template:Chem2 molecules adopt a linear molecular geometry.<ref name="Wells" /><ref name="G&E" />
Uses
Optics
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 magnesium fluoride is one of two materials (the other being lithium fluoride) that will transmit in the vacuum ultraviolet range at 121 nm (Lyman alpha).
Magnesium fluoride is tough and polishes well but is slightly birefringent and should therefore be cut with the optic axis perpendicular to the plane of the window or lens.<ref name=Aigueperse/> Due to its suitable refractive index of 1.37, magnesium fluoride is commonly applied in thin layers to the surfaces of optical elements as an inexpensive anti-reflective coating.Template:Citation needed Its Verdet constant is 0.00810Template:Nbsparcmin⋅G−1⋅cm−1 at 632.8 nm.<ref>J. Chem. Soc., Faraday Trans., 1996, 92, 2753 - 2757. {{#invoke:CS1 identifiers|main|_template=doi}}</ref>
Safety
Chronic exposure to magnesium fluoride may affect the skeleton, kidneys, central nervous system, respiratory system, eyes and skin, and may cause or aggravate attacks of asthma.<ref>Template:Cite web</ref>