Tricarbon: two novel ultra-hard metallic carbon allotropes from first-principle calculations

TL;DR

This paper predicts two new ultra-hard, metallic carbon allotropes, rh-C3 and h-C6, using first-principles calculations, showing they are stable, anisotropic, and possess exceptional mechanical properties.

Contribution

It introduces two novel stable carbon allotropes, rh-C3 and h-C6, characterized by unique crystal structures and superior mechanical properties, expanding the understanding of ultra-hard carbon phases.

Findings

Both phases are energetically stable and comparable to lonsdaleite.

They exhibit extremely high elastic constants and hardness.

Weak metallic behavior is observed in electronic structure calculations.

Abstract

Based on crystal chemistry considerations and quantum density functional theory ground state calculations, rhombohedral rh-C3 and hexagonal h-C6 carbon allotropes are proposed and energetically calculated as new stable ultra-hard phases likewise lonsdaleite. Along the two kinds of carbon in linear C2-C1-C2 lattice, distorted tetrahedra C2(sp3) with an angle of 106.17{\deg} (smaller than ideal 109.4{\deg}) and C1(sp)-like hybridizations are inferred from charge density projections. The calculated elastic constants point to a strong anisotropy of mechanical properties of rh-C3 and h-C6 with an exceptionally large C33 values (1636 GPa and 1610 GPa, respectively), exceeding that of lonsdaleite (1380 GPa), due to the presence of aligned tricarbon units along the hexagonal c-axis. Both phases are characterized by large bulk moduli and high hardness values that are slightly less than those of lonsdaleite and diamond. Weak metallic behavior of both new phases is identified from electronic band structure calculations.