High-Density Ultrahard Novel Orthorhombic Carbon Allotropes C12 and C16 with Physical Properties Close to Diamond: Structural and DFT Investigations

Abstract

Novel orthorhombic carbon allotropes with original topologies: 4⁴T39 C₁₂, mog-C₁₂ and cbs-C₁₆ were devised from crystal structure rationale of C4 tetrahedra stacking and connections backed by density functional theory DFT-based calculations of ground state structures and energy derived physical properties. Specifically, the structures were identified with distorted C4 tetrahedra versus perfect tetrahedra characterizing diamond, accompanied by small atom-averaged volumes resulting into high densities and subsequent ultra hard mechanical behaviors. Dynamically, the allotropes were found stable with positive frequencies revealed from their phonons represented in band structures. Pertaining thermodynamic properties showed specific heat C_V = f(T) calculated curves close to diamond’s experimental values from literature. The closest agreement with experiment was found for the most cohesive allotrope in the series, cbs-C₁₆, concomitantly with the largest electronic indirect band gap, like diamond. From the investigation, a holistic interrelationship: “crystal structure ↔ mechanical ↔ dynamic ↔ electronic properties” is deducted for carbon materials.