An Orthorhombic Phase of Superhard o -BC ₄ N ^*

Abstract

A potential superhard 𝑜-BC4N with 𝐼𝑚𝑚2 space group is identified by ab initio evolutionary methodology using CALYPSO code. The structural, electronic and mechanical properties of 𝑜-BC4N are investigated. The elastic calculations indicate that 𝑜-BC4N is mechanically stable. The phonon dispersions imply that this phase is dynamically stable under ambient conditions. The structure of 𝑜-BC4N is more energetically favorable than 𝑔-BC4N above the pressure of 25.1 GPa. Here 𝑜-BC4N is a semiconductor with an indirect band gap of about 3.95 eV, and the structure is highly incompressible with a bulk modulus of 396.3 GPa and shear modulus of 456.0 GPa. The mechanical failure mode of 𝑜-BC4N is dominated by the shear type. The calculated peak stress of 58.5 GPa in the (100)[001] shear direction sets an upper bound for its ideal strength. The Vickers hardness of 𝑜-BC4N reaches 78.7 GPa, which is greater than that of 𝑡-BC4N and 𝑏𝑐-BC4N proposed recently, confirming that 𝑜-BC4N is a potential superhard material.

Prediction of Novel Ultrahard Phases in the B–C–N System from First Principles: Progress and Problems

“…A similar situation is observed for trigonal ( P 3 m 1) [ 65 ] and orthorhombic ( Imm 2) [ 66 ] BC 4 N, and tetragonal ( P −42 m ) [ 68 ] , rhombohedral ( R 3 m ) [ 68 ] and monoclinic ( Pm and Cm ) [ 69 ] BC 6 N. In all these cases, the use of Gao’s model results in a 4–11% hardness overestimation compared to the values that we obtained in the framework of the thermodynamic model of hardness (see Table 3 ).…”

“…Over the past 20 years, several dozen papers have been published on the subject, but here we will focus only on those that claim the ‘ultrahardness’ ( H V ≥ 75 GPa) of the predicted phases. In addition to different BC 2 N structures [ 35 , 36 , 62 , 63 ], ultrahard phases of BCN [ 64 ], BC 4 N [ 65 , 66 , 67 ], BC 6 N [ 68 , 69 ] and BC 10 N [ 70 ] compositions have been reported.…”

Surperhard monoclinic BC6N allotropes: First-principles investigations*

“…Zhao et al [5] successfully synthesized stoichiometric c-BC 2 N and c-BC 4 N crystals with diamondlike structures by using a diamond-anvil cell. Then Fan et al [6][7][8][9][10][11][12][13][14][15] did systematic investigations of these structures based on the first-principles calculations, such as crystal structure, band gap, optical spectrum, ideal strength and hardness, in order to establish facts and reach new conclusions.…”