Superhard sp3 carbon allotropes with odd and even ring topologies

TL;DR

This paper discovers new superhard sp3 carbon allotropes with unique ring topologies, demonstrating their stability and mechanical properties under high pressure, advancing the understanding of carbon's structural diversity.

Contribution

It introduces four novel sp3 carbon allotropes with specific ring topologies derived via simulations, highlighting their stability and hardness compared to known phases.

Findings

oC16 phase is harder than M-carbon.

A new Cmmm phase is energetically lower than W-carbon.

Mechanical response depends on ring topology.

Abstract

Four sp3 carbon allotropes with six, eight, and 16 atoms per primitive cell have been derived using a combination of metadynamics simulations and topological scan. A chiral orthorhombic phase oC16 (C2221) was found to be harder than monoclinic M-carbon and shows excellent stability in the high-pressure range. A second orthorhombic phase of Cmmm symmetry, by \sim 0.028 eV/atom energetically lower than W-carbon, can be formed from graphite at \sim 9 GPa. In general, the mechanical response under pressure was found to depend on the structure topology, which reflects the way rings are formed from an initial graphene layer stacking.