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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
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 lant ...
and fluorine.
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 inc ...
s 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
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allow ...
.
The larger sized rare earth elements ( lanthanides), 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
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 ...
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, 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% ZrF4, 20% BaF2, 4% LaF3, 3% AlF3 and 20% NaF. ZBLAN is not a single mat ...
. 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 Progr ...
in 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 ...
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 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