Technology Review - This work, like the other structural predictions, is entirely theoretical, relying on computer simulations based on first principle calculations. And until somebody actually measures the structure of this new form of carbon, we won't know which proposal is correct. The process of predicting new carbon allotropes and calculating their properties is itself providing a clear impetus for new research in this field.
Two new carbon allotropes (H-carbon and S-carbon) are proposed, as possible candidates for the intermediate superhard phases between graphite and diamond obtained in the process of cold compressing graphite, based on the results of first-principles calculations. Both H-carbon and S-carbon are more stable than previously proposed M-carbon and W-carbon and their bulk modulus are comparable to that of diamond. H-carbon is an indirect-band-gap semiconductor with a gap of 4.459 eV and S-carbon is a direct-band-gap semiconductor with a gap of 4.343 eV. S-carbon is even more stable than the Z-carbon which is the most table carbon phase proposed recently. The transition pressure from cold compressing graphite is 10.08 GPa and 5.93 Gpa for H-carbon and S-carbon, respectively, which is in consistent with the recent experimental report.
Crystal structure of H-carbon(a), initial AB stacking graphite supercell for H-carbon (b) and side view containing five and seven carbon rings of H-carbon (c). Crystal structure of S-carbon (d), initial AB stacking graphite supercell for Scarbon (e) and side view containing five and seven carbon rings of S-carbon (f).
We propose two new superhard carbon phases, H-carbon and S-carbon, as the possible candidates for the intermediate phase of cold compressing graphite. Both H-carbon and S-carbon with sp3 carbon bonds are optical transparent superhard carbon phases. They are more stable than M-carbon and W-carbon, and S-carbon is the most table carbon phase theoretically proposed until now. Moreover, when the pressure is above 5.93 Gpa (10.08 Gpa), S-carbon (H-carbon) is more favorable than graphite. These two new members together with the previous proposed M-carbon, bct-C4, W-carbon and Z-carbon will enrich the theoretical evidence for understanding the experimental observation.
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