Carbon Nitride Frameworks and Dense Crystalline Polymorphs

TL;DR

This study uses ab initio random structure searching to explore the polymorphism of C3N4 carbon nitride under pressure, discovering new structures, including a stable chiral framework, and predicting phase transitions up to high pressures.

Contribution

It introduces new framework structures of C3N4, including a stable chiral polymorph, and provides a detailed pressure-dependent phase diagram based on ab initio calculations.

Findings

Discovery of a stable chiral P43212 polymorph experimentally produced.

Prediction of a sequence of fully sp3-bonded structures with increasing pressure.

Identification of C3N4 decomposition into diamond and CN2 above 650 GPa.

Abstract

We used ab initio random structure searching (AIRSS) to investigate polymorphism in C3N4 carbon nitride as a function of pressure. Our calculations reveal new framework structures, including a particularly stable chiral polymorph of space group P43212 containing mixed sp2 and sp3-bonding, that we have produced experimentally and recovered to ambient conditions. As pressure is increased a sequence of structures with fully sp3-bonded C atoms and three-fold coordinated N atoms is predicted, culminating in a dense Pnma phase above 250 GPa. Beyond 650 GPa we find that C3N4 becomes unstable to decomposition into diamond and pyrite-structured CN2.