Understanding the nature of "superhard graphite"

TL;DR

This study uses advanced molecular dynamics simulations to identify the most kinetically favorable superhard graphite allotrope formed under cold compression, confirming M-carbon as the final product.

Contribution

It introduces transition path sampling to realistically model nucleation and transformation pathways, revealing the kinetic preference for M-carbon over other structures.

Findings

M-carbon is the kinetically favored final structure.

W-carbon is ruled out due to less probable nucleation.

Bct-C4 is unlikely to form under cold compression.

Abstract

Numerous experiments showed that on cold compression graphite transforms into a new superhard and transparent allotrope. Several structures with different topologies have been proposed for this phase. While experimental data are consistent with these models, the only way to solve this puzzle is to find which structure is kinetically easiest to form. Using state-of-the-art molecular-dynamics transition path sampling simulations, we investigate kinetic pathways of the pressure-induced transformation of graphite to various superhard candidate structures. Unlike hitherto applied methods for elucidating nature of superhard graphite, transition path sampling realistically models nucleation events necessary for physically meaningful transformation kinetics. We demonstrate that nucleation mechanism and kinetics lead to $M$-carbon as the final product. $W$-carbon, initially competitor to $M$-carbon, is ruled out by phase growth. Bct-C$_4$ structure is not expected to be produced by cold compression due to less probable nucleation and higher barrier of formation.