High-energy-density and superhard nitrogen-rich B-N compounds

TL;DR

This study predicts new nitrogen-rich boron nitride compounds with high energy density and superhard properties, stable at ambient conditions, using first-principles calculations and structure searching.

Contribution

It introduces a novel class of B3N5 compounds with unique layered and 3D structures, expanding the potential for high-energy-density materials.

Findings

B3N5 compounds are stable or metastable at ambient pressure.

The C2221-B3N5 phase has an energy density of 3.44 kJ/g.

The predicted hardness is approximately 44 GPa.

Abstract

The pressure-induced transformation of diatomic nitrogen into non-molecular polymeric phases may produce potentially useful high-energy-density materials. We combine first-principles calculations with structure searching to predict a new class of nitrogen-rich boron nitrides with a stoichiometry of B3N5 that are stable or metastable relative to solid N2 and h-BN at ambient pressure. The most stable phase at ambient pressure has a layered structure (h-B3N5) containing hexagonal B3N3 layers sandwiched with intercalated freely rotating N2 molecules. At 15 GPa, a three-dimensional C2221 structure with single N-N bonds becomes the most stable. This pressure is much lower than that required for triple-to-single bond transformation in pure solid nitrogen (110 GPa). More importantly, C2221-B3N5 is metastable, and can be recovered under ambient conditions. Its energy density of 3.44 kJ/g makes it a potential high-energy-density material. In addition, stress-strain calculations estimate a Vickers hardness of 44 GPa. Structure searching reveals a new clathrate sodalite-like BN structure that is metastable under ambient conditions.