Structural, elastic properties and electronic structure of Cr3B4-type borides: An ultra-incompressible material

The structural, elastic properties, hardness and electronic structure of TM3B4 (TM = Cr, Re, Ru and Os) borides are investigated by using first-principles approach. The calculated results show that Re3B4 is more stable than that of other Cr3B4-type borides. These borides all have high bulk modulus and high elastic constant C22 compared with the corresponding diborides. The calculated value of B/G ratio of Cr3B4 (1.404) is lower than 1.75, indicating that this boride exhibits brittle behavior. The calculate bulk modulus of Re3B4 and intrinsic hardness of Cr3B4 is 380 GPa and 38.3 GPa, respectively. The high bulk modulus and hardness derive from the strong B–B and Cr–B covalent bonds along the a–c plane because of the strong charge accumulation between B–B and Cr–B atoms.