Lowest enthalpy polymorph of cold-compressed graphite phase

Abstract

Based on an ab initio evolutionary algorithm, a novel carbon polymorph with an orthorhombic Cmcm symmetry is predicted, named as C carbon, which has the lowest enthalpy among the previously proposed cold-compressed graphite phases.

High-pressure behaviors of carbon nanotubes

“…The proposed M-carbon is the first model to successfully explain the experimental XRD, optical transparency, and superhard nature of cold-compressed graphite [72,73]. After that, a series of models including bct-C 4 [74,75]; W-carbon [76]; structurally equivalent Cco-C 8 [46], Z-carbon [77], oC16-II [78], and Zcarbon-8 [79]; structurally equivalent F-carbon [80], S-carbon [47], Z-carbon-1 [79], and M10-carbon [81]; structurally equivalent O-carbon [82], R-carbon [47], and H-carbon [83,84]; structurally equivalent Z4-A3B1 [85] and P-carbon[47]; structurally equivalent S-carbon [83,84] and C-carbon [86]; X-carbon and Ycarbon [87] were proposed. Recently, the high-pressure experiments and transition path sampling calculations indicate M-carbon is the most likely product of cold compression of graphite [88][89][90][91].…”

Prediction of superhard carbon allotropes from the segment combination method

“…The systems constructed by hybridizing H-diamond and C-diamond in the armchair direction are energy unstable because the interfacial carbon atoms deflect from the stable four bonds configuration. The four low energy allotropes named as Z-CACB [59], Z-ACA [59], S-carbon [67] (S-carbon also named as C-carbon by D. Li et al [61]), and H-carbon [67] (H-carbon also named as O-carbon [60] and R-carbon [63] Fig. 3, a.…”

“…We should point out that such a segment combination method is a special method in searching for low energy superhard carbon allotropes. It is not a universal method of crystal prediction like the particle-swarm optimization method [58,62,69,70], graph theoretical methods [53], evolutionary algorithm USPEX [49,61,64,66], and the minima hopping method [57,65,71] (MHM) for crystal structure prediction [72], which can predict elements crystals from just the chemical composition.…”

“…This novel superhard phase of carbon has led to many theoretical studies and several structures have been proposed for the purpose of fitting the experimental data, such as the sp 2 -sp 3 graphite-diamond structures [47], monoclinic M-carbon [48] (this carbon allotrope was predicated in 2006 [49] and later identified as a candidate [48] for the experimentally observed superhard graphite [45]), body-centered tetragonal C 4 (bct-C 4 ) [50] (this allotrope was predicated in 1997 [51], 2004 [52,53], and 2005 [54] and later identified as a candidate [50] for the experimentally observed superhard graphite [45]), orthorhombic W-carbon [55], and Z-carbon [56][57][58] (Z-carbon is also named as oC16II [58], Cco-C8 [56], and Z4-A 2 B 2 [59]). These theoretical predictions have triggered off many works on searching for low energy superhard carbon allotropes [59][60][61][62][63][64][65][66][67][68]. We find that all these previously proposed superhard carbon allotropes are obtained by the designed computer programs, such as the particle-swarm optimization method [58,62,69,70], graph theoretical methods [53], evolutionary algorithm USPEX [49,61,64,66], and the minima hopping meth...…”

“…These theoretical predictions have triggered off many works on searching for low energy superhard carbon allotropes [59][60][61][62][63][64][65][66][67][68]. We find that all these previously proposed superhard carbon allotropes are obtained by the designed computer programs, such as the particle-swarm optimization method [58,62,69,70], graph theoretical methods [53], evolutionary algorithm USPEX [49,61,64,66], and the minima hopping method [57,65,71] (MHM) for crystal structure prediction [72], or constructed through the schemes reported by Niu [63] and in our latest work [59]. In this paper, we have reviewed these superhard carbon allotropes using segment combination method and found that all of them can be included in two groups: (i) combinations of segments from C-diamond and H-diamond with 5-6-7 carbon rings, and (ii) combinations of segments from H-diamond and mutated H-diamond with 4-6-8 carbon rings.…”