Phase stability, elasticity, hardness and electronic structures for binary MnBm (M = Ni, Cr, Mo, W, n = 23, 5, 3, 1, m = 6, 3, 2, 1) borides: a comprehensive study using first principles

Phase stability, elasticity, hardness and relevant electronic structures of M_nB_m (M = Ni, Cr, Mo, W, n = 23,5,3,1, m = 6,3,2,1) binary borides have been investigated by first-principles calculations. The linear relationships between the formation enthalpies and their boron content x are found in 4 kinds of M_nB_m. The formation enthalpies gradually decrease with the increase of B content. The calculated elastic constants indicate that the M_nB_m (M = Cr, Mo and W) borides change gradually from ductility to brittleness with the increase of B content, while the Ni_nB_m borides show a ductile nature. The sequence in regard to hardness is Cr_nB_m>Mo_nB_m (and W_nB_m) > Ni_nB_m. Electronic structure analysis shows that the variation trend of N(E_F) is in accordance with that of formation enthalpies. Strong M–B bonds and pseudogap between the peaks at both side of the fermi energy are found to play important roles in the hardness and phase stability of these borides.