Magnesium fluoride

Magnesium fluoride is an ionically bonded inorganic compound with the formula MgF2. The compound is a white crystalline salt and is transparent over a wide range of wavelengths, with commercial uses in optics that are also used in 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 can be obtained from the breakdown of ammonium bifluoride:
 * MgO + [NH4]HF2 → MgF2 + NH3 + H2O

Related metathesis reactions are also feasible:
 * Mg(OH)2 + CuF2 → MgF2 + Cu(OH)2

Structure
The compound crystallizes as tetragonal birefringent crystals. The structure of the magnesium fluoride is similar to that of rutile, featuring octahedral Mg(2+) cations and 3-coordinate F− anions.

In the gas phase, monomeric MgF2 molecules adopt a linear molecular geometry.

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). Lower-grade magnesium fluoride is inferior to calcium fluoride in the infrared range.

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. 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. Its Verdet constant is 0.00810arcmin⋅G−1⋅cm−1 at 632.8 nm.

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.