The Local Atomic Structures of Liquid CO at 3.6 GPa and Polymerized CO at 0 to 30 GPa from High-Pressure Pair Distribution Function Analysis

The local atomic structures of liquid and polymerized CO and its decomposition products were analyzed at pressures up to 30 GPa in diamond anvil cells by X-ray diffraction, pair distribution function (PDF) analysis, single-crystal diffraction, and Raman spectroscopy. The structural models were obtained by density functional calculations. Analysis of the PDF of a liquid CO-rich phase revealed that the local structure has a pronounced short-range order. The PDFs of polymerized amorphous CO at several pressures revealed the compression of the molecular structure; covalent bond lengths did not change significantly with pressure. Experimental PDFs could be reproduced with simulations from DFT-optimized structural models. Likely structural features of polymerized CO are thus 4- to 6-membered rings (lactones, cyclic ethers, and rings decorated with carbonyl groups) and long bent chains with carbonyl groups and bridging atoms. Laser heating polymerized CO at pressures of 7 to 9 GPa and 20 GPa resulted in the formation of CO2.