The structure and multifunctionality of high-boron transition metal borides.

High boron content transition metal (TM) borides (HB-TMBs) have recently been regarded as the promising candidate for superhard multifunctional materials. High hardness stem from the covalent bond skeleton formed by high content of boron atoms to resist deformation. High valence electron density of TM and special electronic structure from p-d hybridization of B and TM are the sources of multifunction. However, the reason of hardness variation in different HB-TMBs is still a complex puzzle because hardness is a complex property mainly associated with structures, chemical bonds, and mechanical anisotropy. Rich types of hybridization in boron atom (sp, sp2, sp3) generates abundant structures in HB-TMBs. Studying the intrinsic interaction of structure and hardness or multifunction is significant to search new functional superhard materials. In this review, the stable structure, hardness, and fascinating multifunctionality of HB-TMBs are summarized. It is concluded that the structures of HB-TMBs are mainly composed by sandwiched stacking of B and TM layers. The hardness of HB-TMBs shows a decreasing tendency with the increasing atom radius. The polyhedron in strong B skeleton provides hardness support for HB-TMBs, among which C2/m is the most possible structure to meet the superhard standard. Some HB-TMBs also indicates superconductivity by special structures, most of them have strong hybridization of d electrons from TM near the Fermi level. This work is meaningful to further understand and uncover new functional superhard materials in HB-TMBs.