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Lanthanum trifluoride is a refractory ionic compound of
Bonding is ionic with lanthanum highly coordinated. The cation sits at the center of a trigonal prism. Nine fluorine atoms are close: three at the bottom corners of the trigonal prism, three in the faces of the trigonal prism, and three at top corners of the trigonal prism. There are also two fluorides a little further away above and below the prism. The cation can be considered 9-coordinate or 11-coordinate. At 300 K, the structure allows the formation of Schottky defects with an
lanthanum
Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between l ...
and fluorine
Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reactiv ...
.
The LaF3 structure
Bonding is ionic with lanthanum highly coordinated. The cation sits at the center of a trigonal prism. Nine fluorine atoms are close: three at the bottom corners of the trigonal prism, three in the faces of the trigonal prism, and three at top corners of the trigonal prism. There are also two fluorides a little further away above and below the prism. The cation can be considered 9-coordinate or 11-coordinate. At 300 K, the structure allows the formation of Schottky defects with an activation energy
In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...
of 0.07 eV, and free flow of fluoride ions with an activation energy of 0.45 eV, making the crystal unusually electrically conductive.
The larger sized rare earth elements (lanthanide
The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and yt ...
s), which are those with smaller atomic number, also form trifluorides with the LaF3 structure. Some actinide
The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The info ...
s do as well.
Applications
This white salt is sometimes used as the "high-index" component in multilayer optical elements such asultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
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) Multilayer reflectors and antireflection coatings are typically composed of pairs of transparent materials, one with a low index of refraction
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
, the other with a high index. LaF is one of very few high-index materials in the far UV. The material is also a component of multimetal fluoride glasses such as ZBLAN. It is also doped with europium(II) fluoride in fluoride selective electrodes.
Natural occurrence
LaF3 occurs in the nature as the extremely rare mineral fluocerite-(La). The suffix in the name is known as the Levinson modifier and, by showing the dominant element at a particular site in the structure, is used to differentiate from similar minerals (here: fluocerite-(Ce)).References
Lanthanum compounds Fluorides Lanthanide halides Crystal structure types {{Inorganic-compound-stub