High Pressure Structural Changes in Amorphous Polymeric Carbon Monoxide by Combined Infrared Spectroscopy and X-ray Diffraction

The high pressure transformation of carbon monoxide into an amorphous polymeric material (a-pCO) is known since decades, yet the determination of its local structure is made difficult by the strong photoreactivity of this material along with the high hygroscopicity of the recovered samples. While accurate, nondestructive IR spectroscopy measurements at a few GPa show that a-pCO is a complex solid dominated by the sp2 hybridization for C and with oxygen forming both single and double bonds with carbon, the potential evolution of this chemical structure at higher pressures is hitherto terra incognita. Here we report a high pressure investigation of pCO up to 56 GPa, at room temperature, based on in situ IR spectroscopy and synchrotron X-ray diffraction (XRD) measurements in diamond anvil cells (DACs). Our study shows that this material remains amorphous up to the maximum investigated pressure, with sp2 C substantially converting to sp3 C upon increasing pressure above 12-13 GPa. This occurrence indicates that unsaturated a-pCO modifies with pressure toward an entirely single bonded amorphous material similarly to its theoretically predicted crystalline counterpart.