Ground state structure of polymeric carbon monoxide with high energy density

TL;DR

This study predicts a novel polymeric structure of carbon monoxide with high energy density, surpassing TNT, and explores its phase transitions under pressure using first-principles calculations.

Contribution

It identifies the most stable low-energy structure of CO as a polymeric phase with Pna21 symmetry, challenging the conventional molecular solid view.

Findings

Polymeric Pna21 CO is more stable than molecular form at ambient conditions.

Pna21 transforms into chain-like Cc phase under 7 GPa pressure.

At 100 GPa, CO favors polymerization into a Cmcm phase.

Abstract

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 (4 Carbon + 2 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 chain-like phase of Cc symmetry which has similar ring units to Pna21. On compression to 100 GPa it is energetically favorable for CO to polymerize to form a single-bonded Cmcm phase from another structure of Cmca symmetry composed of units similar to those found in the single-bonded I212121 structure. Thermodynamic stability of these structures was verified using calculations with different density functionals, including hybrid and van der Waals corrected functionals.