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

Abstract

Carbon monoxide (CO) is the first molecular system found to transform into a nonmolecular “polymeric” solid above 5.5 GPa, yet been studied beyond 10 GPa. Here, we show 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, layered phase III. The properties of these phases are consistent with those expected from recently predicted 1D P2<sub>1</sub>/m, 3D I2<sub>1</sub>2<sub>1</sub>2<sub>1</sub>, and 2D Cmcm structures, respectively. Furthermore, the present results advocate a stepwise polymerization of CO triple bonds to ultimately a 2D singly bonded layer structure with an enhanced ionic character.