Borides: Solid‐State Chemistry

Abstract Boron forms compounds with most other elements. In addition to numerous binary and ternary compounds with metals—the true borides—boron forms related compounds with carbon, silicon, nitrogen, phosphorus, arsenic, oxygen, sulfur, and selenium. These related compounds are formed by elements more electronegative than boron and cannot therefore be denoted as borides. They display, however, similar properties and structures as borides, and are, therefore, included in this article. Borides are to some extent related to carbides and nitrides. All three groups of compounds are compounds between small nonmetal and larger metal atoms. They are occasionally denoted interstitial compounds, as the small nonmetal atoms often occupy the interstices between the metal atoms in metal‐rich compounds. In boron‐rich compounds, however, boron octahedra or icosahedra form a continuous three‐dimensional network. Most borides are characterized by high melting points, extreme hardness, brittleness, high chemical stability, and high inertness toward reactive metals. The borides are nonmolecular compounds and knowledge of the crystal structure is consequently of very high importance to characterize a boride phase. The structural principles of mainly the binary and ternary borides are described in this article. There are a total number of at least 1000 binary and ternary borides known at present. The chemical bonding in borides has evident contributions from covalent, ionic as well as metallic bonding, which explains some of the unique properties of the borides. As a rule, phases containing a large proportion of boron are semiconductors, whereas those with a lower proportion of boron are metallic conductors. TiB 2 and MgB 2 (superconductor below 39 K) are good metallic conductors, whereas MeB 6 phases are either semiconductors or metallic conductors depending on the metal. Many theoretical calculations are available, in particular of diborides, including the superconductor MgB 2 . The uses of borides and related compounds are mainly based on their hardness, chemical inertness, and magnetic and electrical properties. A few examples may be mentioned. B 4 C and cubic BN are used as abrasives, B 4 C and hexaborides as surface coatings, and CaB 6 as deoxidation agent in some metallurgical processes. TiB 2 is used in the production of vaporization boats for aluminum. It is also used in the industrial production process for aluminum owing to its high electrical conductivity and inertness toward liquid aluminum. Nd 2 Fe 14 B is, at room temperature, the strongest permanent magnetic material known. Large single crystals of YB 66 are used for monochromating synchrotron radiation.