Hydrogen-Doped Polymeric Carbon Monoxide at High Pressure

The ability to control materials stability, bonding, and transformation by thermo-mechanical and chemical means is significant for development of high-energy-density extended solids. We report that doping hydrogen (∼10%) in carbon monoxide (CO) can greatly lower the polymerization pressure of CO and enhance the stability of recovered polymeric CO products at ambient conditions. Hydrogen-doped CO crystallizes into well-grown dendrites of β-CO-like phase at 3.2 GPa, which polymerizes to highly unsaturated black polymer (phase I) at ∼4.7 (5.8) GPa. Upon further compression, this highly colored polymer transforms into a translucent 3D network structure (phase II) at 6–7 (10–17) GPa and then a transparent 2D layer structure (phase III) at 20–30 (30–60) GPa. A similar series of transformations are also found in pure CO but at considerably higher transition pressures, as noted in parentheses. All polymeric phases are recoverable at ambient conditions, exhibiting an array of phase stability and novel properties such as chemically unstable phase I, highly luminescent phase II, and highly transparent layered phase III. The density of recovered products ranges from ∼2.3 g/cm3 to 3.6 g/cm3, depending on the pressure recovered. The recovered products are highly disordered but slowly decompose to crystalline solids of anhydrous polymeric oxalic acid while exhibiting interesting crystal morphologies such as nm-cobs, nm-lamellar layers, and μm-bales. The present first-principles MD simulations suggest that the polymerization occurs at 6 (or 10) GPa in H2-doped (or pure) CO. While not directly participating in the reaction, the role of H2 molecules is to enhance the mobility of CO molecules leading to the polymerization.