Lanthanum hexaboride
[Image:LaB6HotCathode.jpg|thumb|left|A lanthanum hexaboride hot cathode.]
[Image: Lab6cathodes.jpg|thumb|left|Lanthanum hexaboride cathodes.]
Lanthanum hexaboride 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. 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 are blue. Ion bombardment changes its color from purple to emerald green. LaB6 is a superconductor with a relatively low transition temperature of 0.45 K.