Abstract
CHn is the precursor unit for graphene synthesis. We have theoretically predicated a series of CHn 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 CHn systems. The total density of states for these CHn structures show that only the CH12 phase is metallic above 300 GPa. The rotational properties are traced in H2 and the CHn structures. The CH4 rotation is more sensitive to the pressure. The H2 units are nearly freely rotational. Other structures of CHn, 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 CHn (n > 4) system is a potential candidate for hydrogen storage where H2 could be released by controlling the pressure.
Highlights
IntroductionIn the past few tens of years, a number of studies of Group 14 hydrides were performed; both experimental [2,3,4] and theoretical [5,6,7,8] studies have appeared
We have systematically investigated the CHn crystalline structures with n = 1, 2, 4, 6, 8, 10, and 12 under various pressures by evolutionary algorithms procedures (USPEX)
The mixture of polymer and H2 for CH6, CH10, and CH12 come into play at 400 GPa
Summary
In the past few tens of years, a number of studies of Group 14 hydrides were performed; both experimental [2,3,4] and theoretical [5,6,7,8] studies have appeared These studies were mainly focused on EHx with x ≥ 4 systems (E = Si, Ge, and Sn) which have a high atomic fraction of H (≥80%) and were predicted to metalize as hydrogen-dominant metallic alloys at lower pressures than pure hydrogen. These hydrogen-dominant alloys might become superconductors under high pressure with similar properties to those of pure metallic hydrogen. The strategy for the metallization of hydrogen-dominant metallic alloys is that due to the chemical precompression exerted by heavier atoms on hydrogen, the expected pressure needed for the required metallic transition as a preliminary step toward superconductivity might be within current experimental capabilities
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