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<p>Novel orthorhombic and hexagonal C<sub>12</sub> allotropes with respective topologies <b>unj</b> and <b>umy</b> were devised from crystal chemistry supported by subsequent density functional theory (DFT)-based calculations of ground state structures and energy-derived physical properties. Both allotropes were found with high density magnitude of 3.24 g/cm<sup>3</sup> below diamond (3.55 g/cm<sup>3</sup>) and mechanically stable with large Vickers hardness magnitudes: H<sub>V</sub>(<b>unj</b> C<sub>12</sub>) = 72 GPa and H<sub>V</sub>(<b>umy</b> C<sub>12</sub>) = 45 GPa, letting consider them as ultrahard and superhard respectively. Being dynamically stable with positive phonon frequencies, the two allotropes show relationship trends with diamond experimental C<sub>V</sub> = f(T) heat capacity discreet values. The electronic band structures show insulating&#xa0;properties with large band gap like diamond for <b>unj</b> C<sub>12</sub> and a smaller gap for semi-conducting <b>umy</b>-C<sub>12</sub>. A holistic interrelationship: “crystal structure ↔ mechanical ↔ dynamic ↔ electronic properties” is proposed.</p>

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Novel super hard orthorhombic and hexagonal C12 allotropes: crystal structure rationale and DFT investigations

  • 期刊论文

  • 发表日期：

    2025年8月20日

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  • Samir F. Matar

摘要

Novel orthorhombic and hexagonal C12 allotropes with respective topologies unj and umy were devised from crystal chemistry supported by subsequent density functional theory (DFT)-based calculations of ground state structures and energy-derived physical properties. Both allotropes were found with high density magnitude of 3.24 g/cm3 below diamond (3.55 g/cm3) and mechanically stable with large Vickers hardness magnitudes: HV(unj C12) = 72 GPa and HV(umy C12) = 45 GPa, letting consider them as ultrahard and superhard respectively. Being dynamically stable with positive phonon frequencies, the two allotropes show relationship trends with diamond experimental CV = f(T) heat capacity discreet values. The electronic band structures show insulating properties with large band gap like diamond for unj C12 and a smaller gap for semi-conducting umy-C12. A holistic interrelationship: “crystal structure ↔ mechanical ↔ dynamic ↔ electronic properties” is proposed.