  Ternary C_{x}N_{y}O_{z} compounds are actively researched as novel high
energy density and ultrahard materials. Although some synthesis work has been
performed at ambient conditions, very little is known about the high pressure
chemistry of of C_{x}N_{y}O_{z} compounds. In this work, first principles
variable-composition evolutionary structure prediction calculations are
performed with the goal of discovering novel mixed C_{x}N_{y}O_{z} materials at
ambient and high pressure conditions. By systematically searching ternary
variable composition crystalline materials, the full ternary phase diagram is
constructed in the range of pressures from 0 to 100 GPa. The search finds the
C_{2}N_{2}O crystal containing extended covalent network of C, N, and O atoms,
having space group symmetry Cmc2_{1}, and stable above just 10 GPa. Several
other novel metastable (CO)_{x}-(N)_{y} crystalline compounds discovered during
the search, including two polymorphs of C_{2}NO_{2} and two polymorphs of
C_{3}N_{2}O_{3} crystals are found to be energetically favorable compared to
polymeric carbon monoxide (CO) and nitrogen. Predicted new compounds are
characterized by their Raman spectra and equations of state.
