Novel Super hard Orthorhombic and Hexagonal C12 allotropes: Crystal structure rationale and DFT investigations

Abstract 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 insulation 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.