》 BaF2(Barium fluoride)
Barium fluoride is an inorganic compound with the formula BaF2.
It is a colorless solid that occurs in nature
as the rare mineral frankdicksonite.
Under standard conditions it adopts the fluorite structure and
at high pressure the PbCl2 structure.
Like CaF2, it is resilient to and insoluble in water.
Above ca. 500 °C, BaF2 is corroded by moisture,
but in dry environments it can be used up to 800 °C.
Prolonged exposure to moisture degrades transmission
in the vacuum UV range.
It is less resistant to water than calcium fluoride, but it is the most resistant of all the optical fluorides to high-energy radiation,
though its far ultraviolet transmittance is lower than that
of the other fluorides. It is quite hard, very sensitive to thermal shock and fractures quite easily.
〉 BaF2 properties
Barium fluoride is transparent from the ultraviolet to the infrared, from 150–200 nm to 11–11.5 µm. It is used in windows for infrared spectroscopy, in particular in the field of fuel oil analysis. Its transmittance at 200 nm is relatively low (0.60), but at 500 nm it goes up to 0.96–0.97 and stays at that level until 9 µm, then it starts falling off (0.85 for 10 µm and 0.42 for 12 µm). The refractive index is about 1.46 from 700 nm to 5 µm.
Barium fluoride is also a common, very fast (one of the fastest) scintillators for the detection of X-rays, gamma rays or other high energy particles. One of its applications is the detection of 511 keV gamma photons in positron emission tomography. It responds also to alpha and beta particles, but, unlike most scintillators, it does not emit ultraviolet light. It can be also used for detection of high-energy (10–150 MeV) neutrons, using pulse shape discrimination techniques to separate them from simultaneously occurring gamma photons.
Barium fluoride is used as a preopacifying agent and in enamel and glazing frits production. Its other use is in the production of welding agents (an additive to some fluxes, a component of coatings for welding rods and in welding powders). It is also used in metallurgy, as a molten bath for refining aluminium.
》 MgF2
(Magnesium fluoride)
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 that are also used in space telescopes. It occurs naturally as the rare mineral sellaite.
〉 MgF2 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 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 0.00810 arcmin⋅G–1⋅cm–1 at 632.8 nm.