Dense Carbon Monoxide to 160 GPa: Stepwise Polymerization to Two-Dimensional Layered Solid.

Carbon monoxide (CO) is one of simple molecular systems like N$_{\mathrm{2}}$, O$_{\mathrm{2}}$ and H$_{\mathrm{2}}$, yet been studied at pressures above 5-10 GPa. It is also the first molecular system found to transform into a nonmolecular “polymeric” solid in high energy density at 5.5 GPa; yet, little is known about its structure and transformation beyond this pressure. This imposes a serious short fall in understanding high-pressure behaviors of heteronuclear diatomic systems like CO in comparison with those of homonuclear diatomic systems like N$_{\mathrm{2}}$. Here, we present a series of pressure-induced phase transformations in CO to 160 GPa: from a molecular solid to a highly colored, low-density polymeric phase I to translucent, high-density phase II to transparent, and indirect-gap semi-metallic phase III. The properties of these polymorphs are consistent with those expected from recently predicted \textit{1D} \textit{P2}$_{1}/m,$ \textit{3D} \textit{P2}$_{1}$\textit{2}$_{1}$\textit{2}$_{1}$, and \textit{2D} \textit{Cmcm} structures, respectively. Thus, the present results suggest a stepwise polymerization of CO triple bonds to ultimately a \textit{2D} singly bonded layer structure, as recently found in dense nitrogen (LP-N)