Influences of carbon concentration on crystal structures and ideal strengths of B2CxO compounds in the B-C-O system

Abstract

The search for novel superhard materials with special structures and improved thermal stability and hardness remains considerably experimental and theoretical challenges. Recent reports proposed that higher carbon content in ternary B2CxO compounds, which are isoelectronic with diamond, would lead to increased strength and hardness. This notion was derived from the calculated elastic parameters and empirical hardness formulas based on structural and electronic properties of the equilibrium structures. In present work, we introduce three potential ultra-incompressible and thermodynamically stable B2CxO (x ≥ 2) phases via a systematic particle swarm optimization algorithm structure searches. By evaluating the trends of the crystal configuration, electronic structure, and mechanical properties as a function of the C concentration, it is found that the high carbon concentration benefits the formation of the sp3 C-C covalent bonds and leads to the enhanced elastic moduli and ideal strengths in these B2CxO compounds. Studies of strain-stress behavior at large deformation, however, indicate that all these B2CxO compounds possess substantially lower ideal shear strengths than those of diamond and c-BN, suggesting that they may not be intrinsically superhard.

Superhard B2CO phases derived from carbon allotropes

“…Although evolutionary simulation methods like CALYPSO [16][17][18] and USPEX [19][20][21] have been played a signicant role in successfully predicting superhard materials [4][5][6][7]22,23 all potential crystal structures studied here were constructed manually. Here we conceived and constructed new structures following the steps below: (a) extensively selected carbon allotropes with strong sp 3 C-C covalent bonds; (b) if the original structure contained 4n (n represents integer) atoms, skip step (b), otherwise constructed supercell to make sure the supercell contains 4n atoms; (c) broken symmetry and replaced a quarter of C atoms by O atoms, and a half of C atoms by B atoms.…”

“…6 However all superhard B-C-O phases belong to the tetragonal crystal system. [4][5][6] Our previous work reveals that there exist some other superhard B-C-O phases with non-tetragonal crystal system, as oP8-B 2 CO. 7 Actually, 2-dimensional B-C-O materials have also been studied and proposed as a promising 2-dimensional electronic device materials with tunable band gaps and high carrier mobility. 8 Carbon, which is widely used in industrial applications and scientic research, has numerous allotropes with abundant properties.…”

“…4m2-B 2 C 3 O and P 4m2-B 2 C 5 O. 5 During their research, a reasonable relation between the carbon content and mechanical behavior of B 2 C X O was established and it was suggested that elastic moduli and hardness will benet from the high carbon content. In 2016, Wang et al explored the B-C-O system in a pressure range of 0-50 GPa, and proposed a new superhard phase, B 4 CO 4 , which is metastable under ambient conditions, but thermodynamically stable at high pressures above 23 GPa.…”

“…3) experimentally, great attention has been attracted towards designing superhard materials using B-C-O compounds due to strong sp 3 B-O/C covalent bonds. [4][5][6][7] Li et al studied B 2 CO compounds, which are considered as the simplest ternary B-C-O compounds and are isoelectronic with diamond, and then proposed two new structures, tP4 (tP4-B 2 CO) and tI16 (tI16-B 2 CO), as superhard materials. 4 In Li's opinion, B 2 CO tends to be stable in the form of tetragonal structures with strong sp 3 covalent B-C and B-O bonds.…”

“…Considering that B can combine O/C with strong covalent bonds in solid matter as B-O compounds, [9][10][11][12] and B-C compounds. [13][14][15] Inspired by that tP4-, and tI16-B 2 CO are both diamond-like structures, 4 oP8-B 2 CO is a lonsdaleite-like structure, 7 even the B 2 C X O (X $ 2) phases are also diamond-like structures and can be derived from different tP4-B 2 CO supercells with partial B and O replacing by C. 5 Hence, we are curious about that whether the fabricated B-C-O compounds with replacement of partial C atoms by B and O atoms in carbon allotropes are stable and what properties do they possess.…”