Crystal chemistry and ab initio investigations of ultra-hard dense rhombohedral carbon and boron nitride

TL;DR

This study uses ab initio density functional theory to investigate the crystal chemistry, elastic properties, and electronic structure of ultra-hard rhombohedral forms of carbon and boron nitride, revealing their potential as ultra-hard materials with large bulk moduli and wide band gaps.

Contribution

It provides the first detailed ab initio analysis of rhombohedral dense carbon and boron nitride, including their elastic constants and electronic properties, highlighting their ultra-hard nature.

Findings

Bulk moduli close to diamond and cubic BN.

Both phases classified as ultra-hard materials.

Large band gaps around 5 eV indicating insulating behavior.

Abstract

Rhombohedral dense forms of carbon, rh-C2 (or hexagonal h-C6), and boron nitride, rh-BN (or hexagonal h-B3N3), are derived from rhombohedral 3R graphite based on original crystal chemistry scheme backed with full cell geometry optimization to minimal energy ground state computations within the quantum density functional theory. Considering throughout hexagonal settings featuring extended lattices, the calculation of the hexagonal set of elastic constants, provide results of large bulk moduli i.e. B0(rh-C2) = 438 GPa close to that of diamond, and B0(rh-BN) = 369 GPa close to that of cubic BN. The hardness assessment in the framework of three contemporary models enables both phases to be considered as ultra-hard. From the electronic band structures calculated in the hexagonal Brillouin zones, 3R graphite is a small-gap semiconductor, oppositely to rh-C2 that is characterized by a large band gap close to 5 eV, as well as the two BN phases.