Crystal chemistry and ab initio prediction of ultra-hard rhombohedral B2N2 and BC2N

TL;DR

This paper predicts new ultra-hard rhombohedral B2N2 and BC2N phases using crystal chemistry principles and density functional theory, revealing their structural, electronic, and mechanical properties, including high hardness and insulating behavior.

Contribution

It introduces novel rhombohedral B2N2 and BC2N phases derived from 3R graphite, with detailed computational analysis of their stability and properties, advancing the understanding of ultra-hard materials.

Findings

Rhombohedral B2N2 and BC2N are predicted to be ultra-hard insulators.

Both phases exhibit high bulk and shear moduli.

The predicted hardness values exceed 70 GPa.

Abstract

New ultra-hard rhombohedral B2N2 and BC2N - or hexagonal B6N6 and B3C6N3 - are derived from 3R graphite based on crystal chemistry rationale schematizing a mechanism for 2D => 3D transformation. Full unconstrained geometry optimizations leading to ground state energy structures and energy derived quantities as energy-volume equation of states (EOS) were based on computations within the density functional theory (DFT) with generalized gradient approximation (GGA) for exchange-correlation (XC) effects. The new binary and ternary phases are characterized by tetrahedral stacking alike diamond, visualized with charge density representations, and illustrating ion characters. Atom averaged total energies are similar between cubic BN and rh-B2N2 on one hand, and larger stabilization of rhombohedral BC2N versus cubic and orthorhombic forms (in literature assessed from favored C-C and B-N bonding), on the other hand. The electronic band structures are characteristic of insulators with Egap ~ 5 eV. Both phases are characterized by large bulk and shear moduli and very high hardness values i.e. HV(rh-B2N2) = 74 GPa and HV(rh-BC2N) = 87 GPa.