Physical properties of ReB2 under high pressure and effect of metallic   bonding on its hardness
  Using first-principles calculations, the elastic constants, thermodynamic property and structural phase transition of rhenium diboride under pressure are investigated by means of the pseudopotential plane-waves method, as well as the effect of metallic bond on its hardness. Eight candidate structures of known transition-metal compounds are chosen to probe for rhenium diboride ReB2. The calculated lattice parameters are consistent with the experimental and theoretical values. Based on the third order equation of states, the transition pressure Pt between the ReB2-ReB2 and MoB2-ReB2 phases is firstly determinate. Elastic constants, shear modulus, modulus, ratio and temperature are derived. The single-bonded feather remains in ReB2 compounds.Furthermore, a semiempirical method to evaluate the hardness of multicomponent crystals with partial metallic bond is presented. The effect of metallic bond on the hardness of ReB2 is searched and the hardness shows a gradual increase under compression, which is different from diamond. 
Ming-Min Zhong, Xiao-Yu Kuang, Zhen-Hua Wang, Peng Shao, Li-Ping Ding   and Xiao-Fen Huang
cond-mat.mtrl-sci