Lanthanum hexaboride

Lanthanum hexaboride (LaB6, also called lanthanum boride and LaB) is an inorganic chemical, a boride of lanthanum. It is a refractory ceramic material that has a melting point of 2210 °C, and is insoluble in water and hydrochloric acid. It is extremely hard, with a Mohs hardness of 9.5.[1] It has a low work function and one of the highest electron emissivities known, and is stable in vacuum. Stoichiometric samples are colored intense purple-violet, while boron-rich ones (above LaB6.07) are blue. Ion bombardment changes its color from purple to emerald green.[2] LaB6 is a superconductor with a relatively low transition temperature of 0.45 K.[3]

Lanthanum hexaboride
Names
Other names
Lanthanum boride
Identifiers
ChemSpider
ECHA InfoCard 100.031.379
EC Number
  • 234-531-6
  • InChI=1S/B6.La/c1-2-5(1)3-4(5)6(1,2)3;/q-2;+2 checkY
    Key: VJXOKIULCJPJLW-UHFFFAOYSA-N checkY
Properties
LaB6
Molar mass 203.78 g/mol
Appearance intense purple violet
Density 4.72 g/cm3
Melting point 2,210 °C (4,010 °F; 2,480 K)
insoluble
Structure
Cubic
Pm3m ; Oh
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references
A lanthanum hexaboride hot cathode.
Lanthanum hexaboride cathodes.
Lanthanum hexaboride
Lanthanum hexaboride powdered

Uses

The principal use of lanthanum hexaboride is in hot cathodes, either as a single crystal or as a coating deposited by physical vapor deposition. Hexaborides, such as lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6), have low work functions, around 2.5 eV. They are also somewhat resistant to cathode poisoning. Cerium hexaboride cathodes have a lower evaporation rate at 1700 K than lanthanum hexaboride, but they become equal at temperatures above 1850 K.[4] Cerium hexaboride cathodes have one and half the lifetime of lanthanum hexaboride, due to the former's higher resistance to carbon contamination. Hexaboride cathodes are about ten times "brighter" than tungsten cathodes, and have 10–15 times longer lifetime. Devices and techniques in which hexaboride cathodes are used include electron microscopes, microwave tubes, electron lithography, electron beam welding, X-ray tubes, free electron lasers and several types of electric propulsion technologies. Lanthanum hexaboride slowly evaporates from the heated cathodes and forms deposits on the Wehnelt cylinders and apertures.

LaB6 is also used as a size/strain standard in X-ray powder diffraction to calibrate instrumental broadening of diffraction peaks.[5]

References
  1. Schmidt, Kevin (2014). Computational Modeling of Lanthanum Hexaboride Materials: Interatomic Potentials and Molecular Dynamics (PDF) (Master of Science). University of Nevada, Reno. Retrieved 15 April 2022. Lanthanum hexaboride has a hardness of ≈ 9.5, similar to B4C and between corundum (Al2O3) and diamond on the low and high ends of the Mohs scale, respectively.
  2. T. Lundström (1985). "Structure, defects and properties of some refractory borides" (PDF). Pure and Applied Chemistry. 57 (10): 1383–1390. doi:10.1351/pac198557101383. S2CID 93184983.
  3. G. Schell; H. Winter; H. Rietschel; F. Gompf (1982). "Electronic structure and superconductivity in metal hexaborides". Phys. Rev. B. 25 (3): 1589–1599. Bibcode:1982PhRvB..25.1589S. doi:10.1103/PhysRevB.25.1589.
  4. "Comparing Lanthanum Hexaboride (LaB6) and Cerium Hexaboride (CeB6) Cathodes". Retrieved 2009-05-05.
  5. C. T. Chantler; C. Q. Tran; D. J. Cookson (2004). "Precise measurement of the lattice spacing of LaB6 standard powder by the x-ray extended range technique using synchrotron radiation". Phys. Rev. A. 69 (4): 042101. Bibcode:2004PhRvA..69d2101C. doi:10.1103/PhysRevA.69.042101.
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