Phase stability, hardness and bond characteristics of ruthenium borides from first-principles

Abstract

The structural stability, elastic modulus, hardness and electronic structure of RuB_2−x (0 ≤ x ≤ 2) borides are systematically investigated using a first-principles approach. The calculated results indicate that the boron-poor region is more stable than the boron-rich region. Ru₂B₃ has a higher bulk modulus, shear modulus and Young's modulus compared with RuB₂ and RuB. Moreover, the calculated intrinsic hardness of Ru₂B₃, with a hexagonal structure (space group: P6₃/mmc), is 49.2 GPa, and is therefore a potential superhard material. The high hardness of Ru₂B₃ originates from triangular pyramid bonds, composed of a B–B covalent bond as the base and Ru–B covalent bonds as the two sides. The B–B and Ru–B covalent bonds in the a–c plane resist the applied load, this is the origin of the high elastic modulus and hardness.