Abstract
Two orthorhombic novel superhard sp3-sp2 carbon phases Hcc-C14 and DHcc-C20 are considered theoretically in this study, based on density functional theory. Hcc-C14 and DHcc-C20 allotropes exhibit distinct structural geometries, including C4, C6, and two kinds of C8 carbon rings, containing 14 and 20 carbon atoms in the unit cell, respectively. The electronic band structure calculations of Hcc-C14 and DHcc-C20 show that both allotropes are semiconductors with an indirect bandgap of 2.235 eV and 2.863 eV, respectively. The phonon dispersion under 0 and 100 GPa and calculated elastic constants at 0 GPa prove that these two structures are mechanically and dynamically stable. Also, the hardness of Hcc-C14 and DHcc-C20 is predicted to be 83.7 GPa and 80.9 GPa, which shows the ultra-hardness of these structures. The combination of these outstanding and significant properties indicates these two carbon structures are potential materials for high-frequency electronics, high-temperature, high elastic constants, and for use in aviation and military industry.
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