Two new carbon allotropes (H-carbon and S-carbon) are proposed, as possiblecandidates for the intermediate superhard phases between graphite and diamondobtained in the process of cold compressing graphite, based on the results offirst-principles calculations. Both H-carbon and S-carbon are more stable thanpreviously proposed M-carbon and W-carbon and their bulk modulus are comparableto that of diamond. H-carbon is an indirect-band-gap semiconductor with a gapof 4.459 eV and S-carbon is a direct-band-gap semiconductor with a gap of 4.343eV. The transition pressure from cold compressing graphite is 10.08 GPa and5.93 Gpa for H-carbon and S-carbon, respectively, which is in consistent withthe recent experimental report.
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