First Principles Study on the Stability and Mechanical Properties of MB (M=V, Nb and Ta) Compounds

The first principles calculations based on density functional theory are performed to investigate the stability, chemical bonding, elastic constants, hardness and Debye temperature of MB (M=V, Nb and Ta) compounds. The structures of these borides are optimized, and the lattice parameters are in good agreement with the experimental data. The calculated cohesive energy and formation enthalpy indicate that they are of a thermodynamically stable structure. The mechanical properties, including elastic constants C_ij, bulk modulus, Young's modulus, shear modulus and Poisson's ratio, are calculated. The bulk moduli of them ranging from 263.0 to 278.4 GPa are larger than many common Laves phases and TaB with 278.4 GPa being the largest bulk modulus value among them. The population analysis is used to analyze the chemical bonds in these compounds. The hardness of the compounds is also evaluated, and the result reveals that TaB is the hardest compound among them. The Debye temperature of MB is calculated. The results show that the values of MB compounds range from 419.3 to 794.3 K.