Families of superhard crystalline carbon allotropes induced via cold-compressed graphite and nanotubes

TL;DR

This paper introduces a systematic method to construct families of superhard sp3 carbon allotropes from cold-compressed graphite and nanotubes, explaining their structures and matching experimental data.

Contribution

It proposes a topological analysis-based scheme to generate new superhard carbon allotropes and explains their relation to known structures and experimental observations.

Findings

Predicted two new allotropes, R- and P-carbon, matching experimental diffraction patterns.

Demonstrated these allotropes are wide-gap insulators with hardness comparable to diamond.

Provided a structural framework for understanding phase transformations in compressed graphite and CNTs.

Abstract

We report a general scheme to systematically construct two classes of structural families of superhard sp3 carbon allotropes of cold compressed graphite through the topological analysis of odd 5+7 or even 4+8 membered carbon rings stemmed from the stacking of zigzag and armchair chains. Our results show that the previously proposed M, bct-C4, W and Z allotropes belong to our currently proposed families and that depending on the topological arrangement of the native carbon rings numerous other members are found that can help us understand the structural phase transformation of cold-compressed graphite and carbon nanotubes (CNTs). In particular, we predict the existence of two simple allotropes, R- and P-carbon, which match well the experimental X-ray diffraction patterns of cold-compressed graphite and CNTs, respectively, display a transparent wide-gap insulator ground state and possess a large Vickers hardness comparable to diamond.