Magnesium fluoride
Encyclopedia
Magnesium
fluoride
is an 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
.
The compound crystallizes as tetragonal birefringent crystals. Its structure is similar to that in rutile
, featuring octahedral Mg2+ centers and 3-coordinate fluoride
centres.
Magnesium fluoride can also be prepared from the following simple reaction
Different with other developments, the use of MgCl2 and (NH4)F allows the fabrication of nano to submicrometer MgF2 with controllable morphology (spherical and cubical)
s. Windows, lenses
, and prism
s 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
Thin layers of MgF2 are frequently applied to the surfaces of optical elements as part of optical coating
s such as anti-reflective coating
s.
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
fluoride
Fluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
is an inorganic compound
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
MgF2. The compound is a white crystalline salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
and is transparent over a wide range of wavelengths, with commercial uses in optics
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
.
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
Rutile
Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...
, featuring octahedral Mg2+ centers and 3-coordinate fluoride
Fluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
centres.
Magnesium fluoride can also be prepared from the following simple reaction
- MgCl2 + 2((NH4)F) → MgF2 + 2(NH4Cl)
Different with other developments, the use of MgCl2 and (NH4)F allows the fabrication of nano to submicrometer MgF2 with controllable morphology (spherical and cubical)
Optical properties
Magnesium fluoride is transparent over an extremely wide range of wavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s. Windows, lenses
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
, and prism
Prism (optics)
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
s made of this material can be used over the entire range of wavelengths from 0.120 μm (vacuum ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
) to 8.0 μm (infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
). 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
Lithium fluoride
Lithium fluoride is an inorganic compound with the formula LiF. It is the lithium salt of hydrofluoric acid. This white solid is a simple ionic compound. Its structure is analogous to that of sodium chloride, but it is much less soluble in water. It is mainly used as a component of molten...
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
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
but here it is inferior to calcium fluoride
Calcium fluoride
Calcium fluoride is the inorganic compound with the formula CaF2. This ionic compound of calcium and fluorine occurs naturally as the mineral fluorite . 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...
. MgF2 is tough and works and polishes well, but it is slightly birefringent
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
and should be cut with the optic axis
Optic axis of a crystal
The optic axis of a crystal is the direction in which a ray of transmitted light suffers no birefringence . Due to the internal structure of the crystal , light propagates along the optical axis differently than in other directions...
perpendicular to the plane of the window or lens.
The Verdet constant
Verdet constant
The Verdet constant is an optical "constant" that describes the strength of the Faraday effect for a particular material.The Verdet constant for most materials is extremely small and is wavelength dependent. It is strongest in substances containing paramagnetic ions such as terbium...
of (MgF2) at 632.8 nm is 0.00810 arcmin G−1 cm−1
Thin layers of MgF2 are frequently applied to the surfaces of optical elements as part of optical coating
Optical coating
An optical coating is one or more thin layers of material deposited on an optical component such as a lens or mirror, which alters the way in which the optic reflects and transmits light. One type of optical coating is an antireflection coating, which reduces unwanted reflections from surfaces, and...
s such as anti-reflective coating
Anti-reflective coating
An antireflective or anti-reflection coating is a type of optical coating applied to the surface of lenses and other optical devices to reduce reflection. This improves the efficiency of the system since less light is lost. In complex systems such as a telescope, the reduction in reflections also...
s.
External links
- A java applet showing the effect of MgF2 on a lens
- Infrared windows at Lawrence Berkeley National LaboratoryLawrence Berkeley National LaboratoryThe Lawrence Berkeley National Laboratory , is a U.S. Department of Energy national laboratory conducting unclassified scientific research. It is located on the grounds of the University of California, Berkeley, in the Berkeley Hills above the central campus...
- National Pollutant Inventory - Fluoride and compounds fact sheet
- Crystran Data Crystran MSDS