Lanthanum hexaboride ( La B₆, also called lanthanum boride and LaB) is an inorganic chemical, a boride of lanthanum. It is a

refractory In materials science, a refractory (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are inorganic, non-metallic compound ...

ceramic A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...

material that has a melting point of 2210 °C, and is insoluble in water and

hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...

. It is extremely hard, with a Mohs hardness of 9.5. It has a low

work function In solid-state physics, the work function (sometimes spelled workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here "immediately" ...

and one of the highest

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

emissivities known, and is stable in

vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...

Stoichiometric Stoichiometry () is the relationships between the masses of reactants and products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must equal the total m ...

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. LaB₆ is a superconductor with a relatively low transition temperature of 0.45 K.

Uses

Electron Sources

The principal use of lanthanum hexaboride is in

hot cathode In vacuum tubes and gas-filled tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode, which does not have a heating element ...

s, either as a single crystal or as a coating deposited by

physical vapor deposition Physical vapor deposition (PVD), sometimes called physical vapor transport (PVT), describes a variety of vacuum deposition methods which can be used to produce thin films and coatings on substrates including metals, ceramics, glass, and polym ...

. Hexaborides, such as lanthanum hexaboride (LaB₆) and cerium hexaboride (CeB₆), have low

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 become 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" than

tungsten Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...

cathodes, and have 10–15 times longer lifetime. Devices and techniques in which hexaboride cathodes are used include

electron microscope An electron microscope is a microscope that uses a beam of electrons as a source of illumination. It uses electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing it ...

microwave tube Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequencies between 300 MHz and ...

s, electron lithography,

electron beam welding Electron-beam welding (EBW) is a fusion welding process in which a charged-particle beam, beam of high-velocity electrons is applied to two materials to be joined. The workpieces melt and flow together as the kinetic energy of the electrons is ...

X-ray tube An X-ray tube is a vacuum tube that converts electrical input power into X-rays. The availability of this controllable source of X-rays created the field of radiography, the imaging of partly opaque objects with penetrating radiation. In contras ...

s, free electron lasers and several types of

electric propulsion Spacecraft electric propulsion (or just electric propulsion) is a type of spacecraft propulsion technique that uses electrostatic or electromagnetic fields to accelerate mass to high speed and thus generating thrust to modify the velocity of a ...

technologies. Lanthanum hexaboride slowly evaporates from the heated cathodes and forms deposits on the Wehnelt cylinders and apertures.

X-Ray Diffraction Reference

LaB₆ is also used as an X-ray powder diffraction (XRD or pXRD) peak position and line shape reference standard. It is therefore used to calibrate measured diffractometer angles and to determine instrumental broadening of diffraction peaks. The latter makes crystallite size and strain measurements by XRD possible.

References

{{Borides Lanthanum compounds Borides Refractory materials Ceramic materials