Carbon nitride frameworks and dense crystalline polymorphs

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.