The elasticity, bond hardness and thermodynamic properties of X2B (X=Cr, Mn, Fe, Co, Ni, Mo, W) investigated by DFT theory

The first principles calculations are performed to investigate the elastic constants and thermodynamic parameters of X2B compounds. The calculated elastic constants for these borides indicate that they are mechanically stable structures. The bulk modulus of them range from 230 to 310 GPa and the largest bulk modulus value is attributed to W2B. Furthermore, the scale relation between C44 and bulk modulus is observed in our case. Heat capacity is calculated based on Debye's quasi-harmonic approximation, and we decomposed the heat capacity into two contributions to find that electron excitations dominated the heat capacity at low temperatures. The hardness of X-B and B-B bonds is calculated using a semi empirical hardness theory.