Prediction of a superhard high-pressure phase for CN: First-principles

The energetically favorable crystal structures for stoichiometric ratio as 1:1 (CN) are studied systematically based on the density functional theory combined with the particle-swarm optimization (PSO) method. The predicted favorite phase Pnnm-C4N4 (Pnnm, No. 58) is in excellent agreement with the results of experimental and theoretical studies. The predicted Pnnm-C4N4 phase exhibits the highest bulk modulus and shear modulus of 351 and 335 GPa, respectively. Those are comparable to that of 376 and 390 GPa for c-BN. The calculated phonon dispersions indicate that the Pnnm-C4N4 phase is dynamically stable at ambient conditions, while others require high-pressure condition. The calculated DOS illustrates that the Pnnm-C4N4 phase is an insulator with indirect band gap of 3.746 eV. Moreover, the high hardness of 54 GPa is estimated to the most stable Pnnm-C4N4 phase, indicating it is a potentially superhard material.