Lanthanum trifluoride is a refractory ionic compound of

lanthanum Lanthanum is a chemical element; it has 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 bet ...

and

fluorine Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...

. The chemical formula is .

The LaF₃ structure

Bonding is ionic with lanthanum highly coordinated. The cation sits at the center of a

trigonal prism In chemistry, octahedral molecular geometry, also called square bipyramidal, describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The oc ...

. 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 defect A Schottky defect is an excitation of the site occupations in a crystal lattice leading to point defects named after Walter H. Schottky. In ionic crystals, this defect forms when oppositely charged ions leave their lattice sites and become inco ...

s with an

activation energy In the Arrhenius model of reaction rates, activation energy is the minimum amount of energy that must be available to reactants for a chemical reaction to occur. The activation energy (''E''a) of a reaction is measured in kilojoules per mole (k ...

of 0.07 eV, and free flow of fluoride ions with an activation energy of 0.45 eV, making the crystal unusually

electrically conductive Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...

. The larger sized rare earth elements (

lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises at least the 14 metallic chemical elements with atomic numbers 57–70, from lanthanum through ytterbium. In the periodic table, they fill the 4f orbitals. Lutetium (el ...

s), which are those with smaller atomic number, also form trifluorides with the LaF₃ structure. Some

actinide The actinide () or actinoid () series encompasses at least the 14 metallic chemical elements in the 5f series, with atomic numbers from 89 to 102, actinium through nobelium. Number 103, lawrencium, is also generally included despite being part ...

s do as well.

Applications

This white salt is sometimes used as the "high-index" component in multilayer optical elements such as

ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...

dichroic In optics, a dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths (colours) (not to be confused with dispersion), or one in which light rays having different polarizations are ab ...

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 the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

, 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 ZBLAN is the most stable, and consequently the most used, fluoride glass, a subcategory of the heavy metal fluoride glass (HMFG) group. Typically its composition is 53% Zirconium tetrafluoride, ZrF4, 20% Barium fluoride, BaF2, 4% Lanthanum triflu ...

. It is also doped with

europium(II) fluoride Europium(II) fluoride is an inorganic compound with a chemical formula EuF2. It was first synthesized in 1937.Zhao Yongzhi, Ma Ying, Hou Shaochun, Zhang Wenjuan, Wang Jingjing, Ding Yanrong, Hao Yifan氟化亚铕研究现状 (''lit''. Research Prog ...

fluoride selective electrode A fluoride selective electrode is a type of ion selective electrode sensitive to the concentration of the fluoride ion. A common example is the lanthanum fluoride electrode. Lanthanum fluoride electrode In the lanthanum fluoride electrode, the s ...

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 Levinson is an Ashkenazi Jewish surname meaning "son of Levi". Notable people with the surname include: * Aaron Levinson (born 1963), American producer, musician, composer and record label owner * André Levinson (1887–1933), French dance journ ...

and, by showing the dominant element at a particular site in the structure, is used to differentiate from similar minerals (here: fluocerite-(Ce)).

References

{{Lanthanide halides Lanthanum compounds Fluorides Lanthanide halides Crystal structure types

The LaF3 structure

Applications

Natural occurrence

References

The LaF₃ structure