From layered 2D carbon to 3D tetrahedral original allotropes C12 and C18 with physical properties related to diamond: Crystal chemistry and DFT investigations

TL;DR

This study proposes new 3D carbon allotropes C12 and C18 derived from 2D graphene, demonstrating their stability, mechanical hardness, and electronic properties similar to diamond through DFT calculations.

Contribution

The paper introduces two novel 3D carbon allotropes with unique topologies, stability, and properties, expanding the understanding of carbon's structural diversity.

Findings

C12 and C18 are mechanically and dynamically stable allotropes.

C12 and C18 exhibit high Vickers hardness comparable to diamond.

C18 shows a thermal specific heat curve closely matching experimental diamond data.

Abstract

Two mechanisms of changes from 2D to 3D (D = dimensionality) involving 2D C(sp2) trigonal paving to C(sp3) tetrahedral stacking are proposed through puckering of the 2D layers on one hand and interlayer insertion of extra C on the other hand. Such transformations, led to original 3D hexagonal C12 and C18 allotropes respectively characterized by lon and bac topologies. Using density functional theory DFT calculations, the two allotropes were found cohesive and stable both mechanically (elastic properties) and dynamically (phonons). Comparisons of the physical properties with known uni C6 were established letting identify ranges of large Vickers hardness: HV (uni C6) = 89 GPa, HV (lon C12) = 97 GPa, and HV (bac C18) = 70 GPa. Whilst C6 was identified with acoustic phonons instability, C12 and C18 were found stable dynamically throughout the acoustic and optic frequency ranges. Furthering on the thermal properties the allotropes were characterized with a temperature dependence curve of the specific heat CV close to experimental data of diamond with best fit for novel C18. The electronic band structures reveal a small band gap of 1 eV for uni C6 and larger direct band gap of 3 eV for the two other 3D allotropes. Such modulations of the electronic and physical properties should open scopes of carbon research.