# Novel Superhard Boron Nitrides, B2N3 and B3N3: Crystal Chemistry and First-Principles Studies

## TL;DR

This paper introduces new superhard boron nitride materials, B2N3 and B3N3, using computational methods to predict their stability and hardness.

## Contribution

The paper proposes and validates new boron nitride phases with superhard properties using crystal chemistry and DFT calculations.

## Key findings

- New boron nitrides B2N3 and B3N3 are mechanically and dynamically stable.
- All new phases exhibit superhardness with Vickers hardness exceeding 40 GPa.
- Electronic structures show conductive behavior due to sp2-like π electrons.

## Abstract

Tetragonal and hexagonal hybrid sp3/sp2 carbon allotropes C5 were proposed based on crystal chemistry and subsequently used as template structures to identify new binary phases of the B–N system, specifically tetragonal and hexagonal boron nitrides, B2N3 and B3N3. The ground structures and energy-dependent quantities of the new phases were computed within the framework of quantum density functional theory (DFT). All four new boron nitrides were found to be cohesive and mechanically (elastic constants) stable. Vickers hardness (HV), evaluated by various models, qualified all new phases as superhard (HV > 40 GPa). Dynamically, all new boron nitrides were found to be stable from positive phonon frequencies. The electronic band structures revealed mainly conductive behavior due to the presence of π electrons of sp2-like hybrid atoms.