Ultra-hard rhombohedral carbon from crystal chemistry rationale and first principles

TL;DR

This paper reports the discovery and theoretical analysis of a new ultra-hard rhombohedral carbon phase, rh-C4, derived from graphite, with properties comparable to diamond, using crystal chemistry and first-principles calculations.

Contribution

It introduces a novel ultra-hard rhombohedral carbon structure, rh-C4, constructed via crystal chemistry and analyzed with density functional theory for its stability and properties.

Findings

rh-C4 is an ultra-hard insulator with a 4 eV band gap.

rh-C4 has a higher bulk modulus than lonsdaleite.

rh-C4 exhibits the highest Vickers hardness among studied carbon forms.

Abstract

A new ultra-hard rhombohedral carbon rh-C4 (or hexagonal h-C12) is reported as derived from 3R graphite through crystal chemistry construction and ground state energy within the density functional theory. An extended hexagonal three-dimensional network of h-C12 is formed of C4 tetrahedra alike in h-C4 lonsdaleite (hexagonal diamond). The electronic band structure of rh-C4 is characteristic of insulator with Egap = 4 eV similarly to diamond. From the set of elastic constants a larger value of bulk modulus versus lonsdaleite, and the largest Vickers hardness (HV) versus both forms of diamond were derived.