Lanthanum hexaboride

Lanthanum hexaboride
Names
	Other names Lanthanum boride
Identifiers
CAS Number	12008-21-8 ;
ChemSpider	21241507 ;
ECHA InfoCard	100.031.379
EC Number	234-531-6;
PubChem CID	71308229;
CompTox Dashboard (EPA)	DTXSID601010268 ;
	InChI InChI=1S/B6.La/c1-2-5(1)3-4(5)6(1,2)3;/q-2;+2 Key: VJXOKIULCJPJLW-UHFFFAOYSA-N ;
Properties
Chemical formula	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)
Solubility in water	insoluble
Structure
Crystal structure	Cubic
Space group	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

Lanthanum hexaboride (La B₆, 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 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 LaB_6.07) are blue. Ion bombardment changes its color from purple to emerald green.[1] LaB₆ is a superconductor with a relatively low transition temperature of 0.45 K.[2]

A lanthanum hexaboride hot cathode.

Lanthanum hexaboride cathodes.

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.[3] 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, and free electron lasers. 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.[4]

References

T. Lundström (1985). "Structure, defects and properties of some refractory borides" (pdf). Pure and Applied Chemistry. 57 (10): 1383–1390. doi:10.1351/pac198557101383.
G. Schell; H. Winter; H. Rietschel; F. Gompf (1982). "Electronic structure and superconductivity in metal hexaborides". Phys. Rev. B. 25 (3): 1589–1599. doi:10.1103/PhysRevB.25.1589.
"Comparing Lanthanum Hexaboride (LaB₆) and Cerium Hexaboride (CeB₆) Cathodes". Retrieved 2009-05-05.
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. doi:10.1103/PhysRevA.69.042101.

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[1] T. Lundström (1985). "Structure, defects and properties of some refractory borides" (pdf). Pure and Applied Chemistry. 57 (10): 1383–1390. doi:10.1351/pac198557101383.

[2] G. Schell; H. Winter; H. Rietschel; F. Gompf (1982). "Electronic structure and superconductivity in metal hexaborides". Phys. Rev. B. 25 (3): 1589–1599. doi:10.1103/PhysRevB.25.1589.

[3] "Comparing Lanthanum Hexaboride (LaB₆) and Cerium Hexaboride (CeB₆) Cathodes". Retrieved 2009-05-05.

[4] 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. doi:10.1103/PhysRevA.69.042101.