Structural, mechanical and electronic properties of hafnium borides: a first principle study

International audience ; Transition metal boride, especially the hafnium boride Hf-B, is important and attractive to various applications. Understanding the correlations of material structure to mechanical properties is crucial for designing and synthesizing Hf-B compounds. In this paper, potential structures of the Hf-B system are systematically explored, and their hardness-enhancing and strength failure mechanisms are revealed through first-principle calculations. The results show that hardness of the compounds is estimated to be hard or superhard and is found to be sensitive to the B concentrations and types of sandwich-like structures. For Hf-B with sandwiches II structure, the bulk modulus, shear modulus, Young's modulus, hardness and ideal strength have linear relationship with B concentrations. Strong boron covalent bonding leads to the superhardness of Hf-B compounds with sandwiches I structure and B/(Hf + B) > 0.66. The newly predicted structure C2-HfB6 is estimated to have a superhardness of up to 50 GPa. When applying large deformation, the weakest ideal strengths of the calculated compounds are lower than 20 GPa, due to the breaking of specific Hf-B bonds and the shifting of specific angles of B atoms.