Progress in functional studies of transition metal borides* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFA0401503 and 2018YFA0305700), the National Natural Science Foundation of China (Grant No. 11575288), the Strategic Priority Research Program and Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (Grant Nos. XDB33000000, XDB25000000, and QYZDBSSW-SLH013), and

Abstract

In recent years, transition metal borides (TMBs) have attracted much attention because they are considered as potential superhard materials and have more abundant crystal structures compared with traditional superhard materials. So far, however, no superhard materials have been found in TMBs. A large number of structures and potential new properties in TMBs are induced by the various hybridization ways of boron atoms and the high valence electrons of transition metals, which provide many possibilities for its application. And most TMBs have layered structures, which make TMBs have the potential to be a two-dimensional (2D) material. The 2D materials have novel properties, but the research on 2D TMBs is still nearly blank. In this paper, the research progress of TMBs is summarized involving structure, mechanical properties, and multifunctional properties. The strong covalent bonds of boron atoms in TMBs can form one-dimensional, two-dimensional, and three-dimensional substructures, and the multiple electron transfer between transition metal and boron leads to a variety of chemical bonds in TMBs, which are the keys to obtain high hardness and multifunctional properties of TMBs. Further research on the multifunctional properties of TMBs, such as superconductors, catalysts, and high hardness ferromagnetic materials, is of great significance to the discovery of new multifunctional hard materials.