Decomposition and Polymerization of Solid Carbon Monoxide under Pressure

By performing constant-pressure deformable-cell ab initio molecular dynamics simulations we have studied the pressure-induced chemical instability of CO above 5 GPa. The simulation shows that, contrary to previous speculations, polymerization proceeds without CO bond dissociation. The resulting polymer consists of a disordered network of small polycarbonyl $(\mathrm{CO}{)}_n$ chains connecting fivefold ${\mathrm{C}}_4\mathrm{O}$ cycles. The computed vibrational spectra and electronic gap agree very well with (and shed light on) very recent experimental data.