Structures and properties of functional transition metal borides

Transition metal borides (TMBs) are hard or potential superhard materials due to abrasion resistant, corrosion preventive, oxidation resistance and high hardness. However, few TMBs are superhard materials, so, discussing the strength of TMBs to understand hardness mechanism is necessary. Moreover, there are superconductors, magnetic materials, and catalysts in TMBs. But uncovering more functions in TMBs is important for finding a new kind of functional hard or superhard material. While, high energy is necessary to synthesize TMBs due to strong BB covalent bonds and high melting of transition metal. Thus high temperature or extreme condition is necessary for synthesizing single crystal or bulk sample with high density, which is important for testing physical properties. Various ways of hybridizing boron atoms and high content of valence electron of transition metal are used to induce a large number of structures and potential new properties in TMBs. Boron atoms can form different substructures with different content of boron in TMBs, such as one-dimensional, two-dimensional and three-dimensional (3D) structures. These different boron atom substructures can affect the stability of structure and physical properties, especially hardness, because of the strong covalent bonds between boron atoms. Thus the structure and hardness of TMBs have always received much attention. The multiple electron transfer between transition metal and boron induces diverse chemical bonds in TMBs. All of covalent bonds, ionic bonds, and metal bonds in TMBs determine the mechanic performances, electricitic and magnetic properties, and chemical activity of TMBs. In this work, synthesis method, stability of structure, hardness, and functional properties of TMBs are discussed. The using of high pressure and high temperature is an effective method to prepare TMBs, because under high pressure and high temperature the electrons can transfer between transition-metal atoms and boron atoms in TMBs. There are not only stable TMBs which are even under very high pressure, but also many metastable structures in TMBs. Hardness values of TMBs are discussed by different content of boron, the high boron content or even 3D boron structure is not superhard material. Because insufficient electron transfer can form the distorted BB covalent bond which is weaker than directional covalent bonds like CC in diamond. Thus electron transfer is significant in TMBs for designing hard or even superhard materials. Besides high hardness, there are superconductor, magnetic material, and catalyzers in TMBs, but there are many potential properties of TMBs which are unknown. Further study to uncover the new properties of TMBs is significant for finding a new kind of functional hard material.