Theoretical Prediction of CH_n Crystal Structures under High Pressures

CH_n is the precursor unit for graphene synthesis. We have theoretically predicated a series of CH_n structures with n = 1, 2, 4, 6, 8, 10, and 12 at elevated pressures (ambient pressure, 50, 100, 200, 300, 350, and 400 GPa) using evolutionary algorithms. The predicted CH and CH₂ structures are graphane-type and polyethylene over the whole considered pressure range, respectively. The molecular crystalline methane is predicted for the stoichiometry of CH₄. The combination of methane and H₂ for CH₆, CH₈, CH₁₀, and CH₁₂ up to 300 GPa are obtained. At 400 GPa, the mixture of polymer and H₂ for CH₆, CH₁₀, and CH₁₂ comes into play. From the computed enthalpy, higher pressure and more hydrogen concentration contributed to the decomposition (to carbon and H₂) of CH_n systems. The total density of states for these CH_n structures show that only the CH₁₂ phase is metallic above 300 GPa. The rotational properties are traced in H₂ and the CH_n structures. The CH₄ rotation is more sensitive to the pressure. The H₂ units are nearly freely rotational. Other structures of CH_n, including fcc-type and experimentally known structures, are not competitive with the structures predicted by evolutionary algorithms under high pressure region. Our results suggest that the CH_n (n > 4) system is a potential candidate for hydrogen storage where H₂ could be released by controlling the pressure.