Lanthanum trifluoride is a refractory ionic compound of lanthanum and fluorine.
Molar mass： 195.900 g/mol
Appearance： white, crystalline solid
Density： 5.9 g/cm3
Melting point： 1,493 °C (2,719 °F; 1,766 K)
Refractive index (nD)：1.606
Crystal structure：Rhombohedral, hR24
Lanthanum fluoride is sometimes used as the “high-index” component in multilayer optical elements such as ultraviolet dichroic and narrowband mirrors. Fluorides are among the most commonly used compounds for UV optical coatings due to their relative inertness and transparency in the far ultraviolet (FUV, 100 nm <λ <200 nm). Multilayer reflectors and antireflection coatings are typically composed of pairs of transparent materials, one with a low index of refraction, the other with a high index. There are very few high-index materials in the far UV. LaF3 is one. The material is also a component of multimetal fluoride glasses such as ZBLAN. It is also used (with europium fluoride, EuF2) in fluoride selective electrodes.