Hexaboride

Hexaboride is a class of binary inorganic compounds with the general formula MB6, where M is a metal element such as a rare-earth, alkaline-earth, or transition metal. In these materials boron atoms form clusters of six atoms (B6) that assemble into a rigid covalent framework, while the metal cation occupies interstitial positions to balance charge. Hexaborides are known for their high melting points, chemical stability, and, in some cases, hardness, properties that are favorable in vacuum environments.

Crystal structure and bonding: The boron sublattice consists of B6 octahedra connected in a three-dimensional network; metal ions inhabit interstitial sites. The bonding is largely covalent within the boron framework with metallic character contributed by the embedded metal. The interplay between the covalent boron network and the electron donation from the metal governs the electronic structure and physical properties of each hexaboride.

Notable members and properties: Lanthanum hexaboride (LaB6) is widely used as a thermionic electron emitter because of its relatively low work function and high-temperature stability, enabling efficient electron sources for electron microscopes and other vacuum tubes. Cerium hexaboride (CeB6) is a mixed-valence compound with interesting magnetic and electronic properties; yttrium hexaboride (YB6) is superconducting at low temperatures. Other hexaborides such as calcium hexaboride (CaB6) have been studied for their electronic structure and potential semiconducting behavior. The family displays a range of magnetic, electronic, and superconducting phenomena depending on the metal.

Synthesis and processing: Hexaborides are typically prepared by high-temperature reaction of the metal with boron under inert atmosphere or vacuum, or by reduction methods from borates or oxides. The resulting MB6 powders or crystals can be consolidated into ceramics or used directly as boron-rich materials for various applications.

Applications and research: Hexaborides are studied for thermionic emitters, coating materials, and as model systems in solid-state chemistry and condensed-matter physics. LaB6 is a common cold cathode in electron-optical instruments; CeB6 and related compounds are platforms for exploring magnetism and correlated electron behavior.