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Optical Thin-Film Coatings
Published in Paul Klocek, Handbook of Infrared Optical Materials, 2017
Magnesium fluoride with an index of 1.37–1.38 is a durable nonreactive coating material used for single-layer AR coatings in the visible and near-infrared. The most durable coatings are obtained with magnesium fluoride when it is deposited on glass heated to at least 275°C. If deposited at lower temperatures, the coating becomes less dense, lower in index, and soft. At higher temperatures, magnesium fluoride can be deposited in thicknesses to use as SLAR coatings out to about 4.0 μm. At wavelengths greater than 4.0 μm, stresses build up in the coating that cause it to crack and delaminate from the substrate. Recently, ion-assisted deposition (IAD) has been used to deposit magnesium fluoride at lower temperatures, even as low as ambient room temperature. The results have been encouraging and indicate that it may be possible in the future to develop processes involving IAD to make durable magnesium fluoride coating at low substrate temperatures.
Material Properties
Published in H. Angus Macleod, Thin-Film Optical Filters, 2017
The material probably used more than any other in thin-film work is magnesium fluoride. This has an index of approximately 1.38 or 1.39 in the visible region (see Figure 14.1) and is extensively used in the antireflection of lenses. In the simplest case, this is a single layer. Early workers used fluorite (CaF2), but this was found to be rather soft and vulnerable and was subsequently replaced by magnesium fluoride. Magnesium fluoride can be evaporated from a tantalum or molybdenum boat, and the best results are obtained when the substrate is hot at a temperature of some 200–300°C. When magnesium fluoride is evaporated, trouble can sometimes be experienced through spitting and flying out of material from the boat. This is thought to be caused by thin coatings of magnesium oxide round the grains of magnesium fluoride in the evaporant. Magnesium oxide has a rather higher melting point than magnesium fluoride, and the grains tend to explode once they have reached a certain temperature. It is important, therefore, to use a reasonably pure grade of material, preferably one specifically intended for thin-film deposition and, especially, to protect it from atmospheric moisture.
Physical properties of magnesium fluoride: FPLAPW Approach
Published in Phase Transitions, 2023
Abdeldjalil Arroussi, Mohamed Ghezali, Abdelbasset Baida, Belkacem Mahfoud, Benameur Amiri
Alkaline earth metal fluorides include MgF2 crystallizes in the rutile structure [4–7] while CaF2, SrF2 and BaF2 crystallize in the fluorite-type structure [8–11]. Magnesium fluoride crystallizes under ambient conditions in a rutile-like tetragonal structure with a space group of P42/mnm and is isomorphic to rutile and stishovite. The phase transition of Mgf2 is obtained at a lower pressure than many dioxides. This is due to the high compressibility of this material and the large relative size of the ions. These phase transitions can be investigated under hydrostatic or quasi-hydrostatic conditions [12].