The Phase Diagram and Hardness of Carbon Nitrides

TL;DR

This paper presents a comprehensive phase diagram of carbon nitrides, discovering three new superhard, thermodynamically stable phases and analyzing their structures and hardness, with implications for replacing diamond in demanding applications.

Contribution

It provides the first systematic phase diagram of the C-N system at 0-300 GPa and identifies new stable superhard phases through evolutionary structure searches.

Findings

Discovered three new superhard, thermodynamically stable carbon nitride phases.

Found that the previously proposed graphitic-C3N4 structure is dynamically unstable at zero pressure.

Identified that the hardest metastable structures are based on s-triazine and s-heptazine units.

Abstract

Novel superhard materials, especially those with superior thermal and chemical stability, are needed to replace diamond. Carbon nitrides (C-N), which are likely to possess these charac- teristics and have even been expected to be harder than diamond, are excellent candidates. Here we report three new superhard and thermodynamically stable carbon nitride phases. Based on a systematic evolutionary structure searches, we report a complete phase diagram of the C-N system at 0-300 GPa and analyze the hardest metastable structures. Surprising- ly, we find that at zero pressure, the earlier proposed graphitic-C3N4 structure (P-6m2) is dynamically unstable and the lowest energy form of structures based on s-triazine unit and s-heptazine unit have similar topology and belong to the same space group.