- Lanthanum hexaboride
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IUPACName =
OtherNames = lanthanum boride
Section1 = Chembox Identifiers
CASNo = 12008-21-8
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Section2 = Chembox Properties
Formula = LaB6
MolarMass = 203.776 g/mol
Appearance = intense purple violet
Density =
MeltingPt = 2210 °C
BoilingPt =
Solubility = insoluble
Section3 = Chembox Hazards
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Autoignition =Lanthanum hexaboride (LaB6, also called lanthanum boride and LaB) is an inorganic chemical, a
boride oflanthanum . It is arefractory ceramic material that has a melting point of 2210 °C, is insoluble in water andhydrochloric acid . It has a lowwork function and one of the highestelectron emissivities known, and is stable invacuum . ItsCAS number is CASREF|CAS=12008-21-8.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. [http://www.iupac.org/publications/pac/1985/pdf/5710x1383.pdf]The principial use of lanthanum hexaboride is in
hot cathode s, either as a single crystal or as a coating deposited byphysical vapor deposition . Hexaborides, such as Lanthanum hexaboride (LaB6) andcerium hexaboride (CeB6), have lowwork function s, 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 becomes equal at temperatures above 1850 K. 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" thantungsten cathodes, and have 10-15 times longer lifetime. They are used eg. inelectron microscope s,microwave tube s,electron lithography ,electron beam welding ,X-Ray tube s, andfree electron laser s.Lanthanum hexaboride slowly evaporates from the heated cathodes and forms deposits on the
Wehnelt cylinder s and apertures. [http://www.microscopy.com/cgi-bin/ReadPrintEmailHTML.pl?filename=9906.txt]
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