Structural, mechanical and electronic properties of Nb₂C: First-principles calculations

Nb-C compounds are potential candidates to achieve high hardness and refractory nature. We performed a crystal structure search for the Nb-C system using an ab initio evolutionary algorithm implemented in the USPEX code. By comparing the formation enthalpy, a P-x phase diagram was calculated, and an orthorhombic Pnnm structure of Nb₂C was predicted and denoted as Nb₂C-I, which was both mechanically and dynamically stable. In this Pnnm phase, there are four-sided rings continuously along the c-axis, which probably contributes to the relatively high incompressibility of Nb₂C-I along the crystallographic c-axis. Moreover, the hardness and Young's modulus were calculated to be 28.5 GPa and 448.9 GPa, respectively; hence, the Nb₂C-I is considered as a potential ultra-stiff and hard material.