First-principles simulation of cyanogen under high pressure: Formation of paracyanogen and an insulating carbon nitride solid

Using a set of first-principles calculations, we have studied the phase transitions of cyanogen molecules (C₂N₂) under high pressure. We obtained that at high pressure, cyanogen is transformed to a planar graphitelike structure with metallic property, which is experimentally known as paracyanogen. By increasing pressure, paracyanogen is transformed to an insulator carbon-nitrogen solid with an indirect band gap of 3.2 eV. Both above-mentioned phase transitions are first order and occur simultaneously with significant reduction in volumes. From our calculations, it is found that the planar structure of paracyanogen is made of ten-membered rings consisting of two C-C, four C-N, and four C=N bonds, while each carbon atom has sp2-like hybridization. In the new-formed solid, all the carbon atoms have sp3-like hybridization and all the nitrogen atoms have sp2-like hybridizations with their nearest-neighboring C and N atoms. Our optical calculations show that cyanogen, which is a colorless gas, after above transitions turns to black paracyanogen polymer and then to a transparent solid. We have also used phonon dispersion calculation to show the stability of our predicted three-dimensional carbon-nitrogen system. Furthermore, it turns out that the predicted CN solid with stoichiometry 1:1 has also a good elastic property and a high bulk modulus of 330 GPa.