Pressure-induced structure, elasticity, intrinsic hardness and ideal strength of tetragonal C4N

Abstract

A tetragonal C₄N (t-C₄N) structure was predicted via CALYPSO code, and the effects of pressure on its structural and mechanical properties were studied. The results show that t-C₄N is different from various 2D C_xN_y compounds with a new type 3D crystal structure, which is similar to diamond. Bulk t-C₄N is equipped with excellent elastic properties. When the pressure is increased from 0 GPa to 350 GPa, its bulk modulus B, shear modulus G and Young's modulus E are increased from 426.9 GPa to 1123.1 GPa, 371.4 GPa to 582.9 GPa and 863.7 GPa to 1490.9 GPa, respectively. The anisotropic B_max, G_max and E_max are increased from 582.38 GPa to 1751.41 GPa, 478.29 GPa to 1033.97 GPa and 1281.26 GPa to 2490.14 GPa, respectively. When the pressure is 0 GPa, the hardness calculated by Chen's and Tian's models are 51.15 GPa and 51.81 GPa, respectively. Its ideal tensile strength in [111] orientation is the smallest (63.46 GPa), which indicates that the (111) planes allow easy cleavage. The smallest ideal shear strength (67.98 GPa) can be obtained in the (111)[10] orientation, which suggests its theoretical hardness is about 67.98 GPa. Due to its excellent mechanical properties, t-C₄N can be used as an industrial superhard material.