Metal Borides: Versatile Structures and Properties

Abstract

Abstract Borides represent an important class of inorganic solids. Often they exhibit interesting properties and can be used either as structural and functional materials or as catalysts. Boron is a non‐metallic, light element with three valence electrons. Its ability to form multi‐centred bonds to overcome the electron deficiency is huge, and it manifests itself in an enormous variety of complicated structures. Many metals react with boron and form borides. Metal borides (MxBz, MxM′yBz, etc.; M, M′ = metals) are binary or multinary compounds in which boron is the most electronegative partner. The huge variety of boride structures is known. It is reasonable to distinguish between, on the one hand, the more metal‐rich borides in which boron atoms are either isolated or they form dumbbells or chains and, on the other side, the boron‐rich materials that display very exotic framework structures, often consisting of beautiful deltahedra. In between there are the layered structures with two‐dimensional arrays of boron atoms. The degree of dimensionality of the arrangement of the boron atoms correlates closely with the (non)metallic character of the borides. Thus, the structural diversity is reflected by an extreme variability of electrical properties between metallic and ceramic. The huge influence of the boron atom structure on the electronic structure of the metal borides is quite unique. It has its origin in the special electron‐deficient character of the boron atom that often leads to a very special bonding situation in boron compounds.