Cerium hexaboride

Cerium hexaboride

Names
Other names cerium boride, ceBIX, CEBIX
Identifiers
CAS Number	12008-02-5 Y
ECHA InfoCard	100.031.375
EC Number	234-526-9
InChI InChI=1/B6.Ce/c1-2-3(1)5(1)4(1,2)6(2,3)5;/q-2;+2
Properties
Chemical formula	CeB6
Molar mass	204.986 g/mol
Density	4.80 g/cm3
Melting point	2,552 °C; 4,625 °F; 2,825 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

Cerium hexaboride (CeB₆, also called cerium boride, CeBix, CEBIX, and (incorrectly) CeB) is an inorganic chemical, a boride of cerium. It is a refractory ceramic material. It has low work function and one of the highest electron emissivity known, and is stable in vacuum. The principal use of cerium hexaboride is a coating of hot cathodes, or hot cathodes made of cerium hexaboride crystals. It usually operates at temperature of 1450 °C.

A cerium hexaboride top hat cathode. Image courtesy of Applied Physics Technologies

Lanthanum hexaboride (LaB₆) and cerium hexaboride (CeB₆) are used as coating of some high-current hot cathodes. Hexaborides show low work function, around 2.5 eV. They are also somewhat resistant to cathode poisoning. Cerium boride cathodes show lower evaporation rate at 1700 K than lanthanum boride, but it becomes equal at 1850 K and higher above that. Cerium boride cathodes have one and half the lifetime of lanthanum boride, due to its higher resistance to carbon contamination. Boride cathodes are about ten times as "bright" than the tungsten ones and have 10–15 times longer lifetime. In some laboratory tests, CeB₆ has proven to be more resistant to the negative impact of carbon contamination than LaB₆. They are used e.g. in electron microscopes, microwave tubes, electron lithography, electron beam welding, X-Ray tubes, and free electron lasers.

Cerium hexaboride, like lanthanum hexaboride, slowly evaporates during the cathode operation. In conditions where CeB₆ cathodes are operated under 1850 K, CeB₆ should maintain its optimum shape longer and therefore last longer. While the process is about 30% slower than with lanthanum boride, the cerium boride deposits are reported to be more difficult to remove.^[1]

References