Lanthanum hexaboride
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Lanthanum hexaboride (LaB6, also called lanthanum boride and (incorrectly) LaB) is an inorganic chemical, a boride of lanthanum. It is a refractory ceramic material. It is an odorless powder with melting point of 2210 °C, insoluble in water and hydrochloric acid. It is slightly hygroscopic. It has low work function and one of the highest electron emissivity known, and is stable in vacuum. Its CAS number is [12008-21-8][link]. Stoichiometric samples are colored intense purple-violet, boron-rich ones (above LaB6.07 are blue. [link]
The principial use of lanthanum hexaboride is a coating of hot cathodes, or hot cathodes made of lanthanum hexaboride crystals. The coatings are usually deposited by physical vapor deposition. Ion bombardment changes its color from purple to emerald green.
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. They are used eg. in 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 pesky deposits on the Wehnelt cylinders and apertures. [link]
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