In this section, the elemental forms of rare-earth elements are iron gray to silvery lustrous metals that are typically soft, malleable, ductile, and usually reactive, especially at elevated temperatures or when finely divided. rare-earth elements are examined in terms of physical and chemical properties. This makes them essential components of diverse defense, energy, industrial, military technology, and low-carbon technologies. Furthermore, REEs are rapidly being used in magnet applications. For example, magnets produced by Neodymium-iron, the strongest known type of magnet, are used widely. Thus, their application areas vary from the electronic to glass industry. Also, information about the sources of rare-earth elements is given in this part.

Rare-earth hexaborides (REB6) are composed of rare-earth elements and octahedral 3D boron units. In Chapter 1, rare-earth elements were examined in detail; in this part, the REB6 will be explained. Hence, rare-earth hexaborides (REB6) consisting of rare-earth elements and octahedral bor units are a group of ceramic materials that have a simple cubic structure with Pm3m symmetry. Their low electronic work function, low electrical resistance, and thermal expansion coefficient (in some temperature ranges), as well as high hardness and stiffness, high chemical and thermal stability, and melting points, provide a wide range of industrial uses from metallurgy to electronics.

The structures of rare-earth hexaborides can be nanoparticles, nanowires, nanotubes, nanorods, nano-obelisks, nanocubes, nanocrystals and nanocons. These types of structures indicate superior properties, such as excellent mechanical, electronic, and optical properties. For these reasons, they are used in thermionic materials, electrical coating for resistors, sensors, and high-energy optical systems. Furthermore, their low work functions make them special for the design of optical devices, such as a cathode substance for cold (field) emission

To produce rare-earth hexaborides, some methods exist: direct solid phase, carbothermal reduction, borothermal reduction, self-propagating synthesis, aluminum flux method, spark plasma sintering, and mechanochemical synthesis, floating zone method, and chemical vapor deposition. In this section, the drawbacks and advantages of these production methods will be discussed.

The rare-earth hexaboride can be both alloyed with alkaline earth hexaboride and rare-earth hexaborides. Both alloying types have different types of advantages. For example, large-size triple LaxCe1-xB6 single crystals produced by the floating zone method showed excellent field emission and thermionic emission characteristics. Thus, these types of alloys indicate superior performance (electronic, magnetic, excellent field emission, thermionic emission properties) when compared to their pure counterparts. 

Rare-Earth metal hexaborides (REB6) can be composited with some kind of ceramics, such as SiC, MgO, Carbon Nanotube, and Alumina. These types of composites can show excellent mechanical, optical, and thermionic properties. For example, SiC ceramics have high condensation behavior, high corrosion resistance, high thermal shock resistance, and high hardness properties; MgO ceramics have high fire resistance, high thermal conductivity, and low electrical conductivity properties; Carbon nanotubes have high optical and mechanical properties and Al2O3 ceramics have high abrasion and corrosion resistance and low density. The sizes of these materials are also significant as nano, and micro-sized ceramic materials have different properties when forming a composite with REB6 or any materials.