Systematic search for low-enthalpy sp3 carbon using evolutionary metadynamics

TL;DR

This paper uses evolutionary metadynamics to systematically discover low-enthalpy metastable superhard carbon allotropes, including novel structures with unique topologies, expanding the understanding of carbon phases accessible from graphite under pressure.

Contribution

It introduces evolutionary metadynamics as an effective method to find global minima and metastable phases in carbon, predicting new allotropes with unique topologies from graphite.

Findings

Identified all known superhard carbon candidates.

Predicted two novel allotropes: X-carbon and Y-carbon.

Classified superhard allotropes into five topology families.

Abstract

We present a systematic search for low-energy metastable superhard carbon allotropes by using the recently developed evolutionary metadynamics technique. It is known that cold compression of graphite produces an allotrope at 15-20 GPa. Here we look for all low-enthalpy structures accessible from graphite. Starting from 2H- or 3R-graphite and applying the pressure of 20 GPa, a large variety of intermediate $sp^3$ carbon allotropes were observed in evolutionary metadynamics simulation. Our calculation not only found all the previous proposed candidates for `superhard graphite', but also predicted two allotropes (\emph{X}-carbon and \emph{Y}-carbon) showing novel 5+7 and 4+8 topologies. These superhard carbon allotropes can be classified into five families based on 6 (diamond/lonsdaleite), 5+7 (\emph{M/W}-carbon), 5+7 (\emph{X}-carbon), 4+8 (bct C$_4$), and 4+8 (\emph{Y}-carbon) topologies. This study shows evolutionary metadynamics is a powerful approach both to find the global minima and systematically search for low-energy metastable phases reachable from given starting materials.