National Research Council of Canada. Security and Disruptive Technologies
Crystal structure prediction methods and first-principles calculations have been used to explore low-energy structures of carbon monoxide (CO). Contrary to the standard wisdom, the most stable structure of CO at ambient pressure was found to be a polymeric structure of Pna21 symmetry rather than a molecular solid. This phase is formed from six-membered (four carbon + two oxygen) rings connected by C=C double bonds with two double-bonded oxygen atoms attached to each ring. Interestingly, the polymeric Pna21 phase of CO has a much higher energy density than trinitrotoluene (TNT). On compression to about 7 GPa, Pna21 is found to transform into another chainlike phase of Cc symmetry which has similar ring units to Pna21. On compression to 12 GPa, it is energetically favorable for CO to polymerize into a purely single bonded Cmca phase, which is stable over a wide pressure range and transforms into the previously known Cmcm phase at around 100 GPa. Thermodynamic stability of these structures was verified using calculations with different density functionals, including hybrid and van der Waals corrected functionals.