Magnesium fluoride (MgF₂) is a tetragonal crystal with a rutile-type structure, having lattice parameters of a = 4.64 Å and c = 3.06 Å. Its point group is 4/mmm, and its space group is P4₂/mnm. When exposed to ultraviolet radiation or electron-beam excitation, it exhibits faint purple fluorescence. MgF₂ crystals possess excellent optical and polarising properties, making them particularly suitable for ultraviolet and infrared spectroscopy. They are transparent over a broad wavelength range (110 nm–7.5 μm) and are therefore widely used in space telescope optics. Single crystals are also employed in vacuum-ultraviolet-transparent windows, polarising prisms, and optical lenses.
Magnesium fluoride thin films exhibit a low refractive index (n ≈ 1.38), a broad transmission range (120–8,000 nm), a wide bandgap (≈11 eV), excellent thermal stability, high mechanical strength, and a high laser-induced damage threshold. These properties make MgF₂ an important material for the fabrication of anti-reflective coatings, high-reflectivity coatings, and high-damage-threshold optical coatings. Consequently, MgF₂ thin films are widely used in a variety of optical coating applications.
