Stable and Metastable Mixed Polymeric Carbon, Nitrogen, and Oxygen Compounds at High Pressures

Polymeric C$_{\mathrm{x}}$N$_{\mathrm{y}}$O$_{\mathrm{z}}$ compounds are promising candidates for novel high energy density materials. Both nitrogen and carbon monoxide transform into polymeric high energy density materials at high pressures: over 100 GPa for nitrogen and just over a few GPa for polymeric carbon monoxide (p-CO). The recovery of polymeric nitrogen at ambient conditions remains problematic while p-CO is found to decompose at ambient conditions. In spite of the potential usefulness of C$_{\mathrm{x}}$N$_{\mathrm{y}}$O$_{\mathrm{z}}$ compounds, very little is known about their high pressure chemistry. In this work, extensive first principles variable-composition evolutionary structure prediction calculations are performed to predict the mixed C$_{\mathrm{x}}$N$_{\mathrm{y}}$O$_{\mathrm{z}}$ phase diagram at pressures up to 100 GPa. The search reveals the polymeric C$_{\mathrm{2}}$N$_{\mathrm{2}}$O structure in the space group Cmc2$_{\mathrm{1}}$, which is a known structure of Si$_{\mathrm{2}}$N$_{\mathrm{2}}$O, to be stable at just 10 GPa. We also predict several metastable mixed (CO)$_{\mathrm{x}}$-(N$_{\mathrm{2}})_{\mathrm{y}}$ structures energetically favorable compared to p-CO and N$_{\mathrm{2}}$. Several materials are predicted to have an energy density comparable to p-CO at ambient conditions. Predicted structures are characterized by their Raman spectra and equations of state.