High-density superhard carbon stabilized by strain induced phase-transition and light absorption enhancement of its low-dimensional graphyne under high-pressure

By employing crystal structure prediction techniques, we introduce two novel carbon allotrope which exhibits Ima2 (orthorhombic, designated as oI32) and C2/m symmetry (monoclinic, termed as mC218), respectively. Comprehensive phonon dispersion analyses reveal the inherent dynamic instability of mC218. Intriguingly, the strategic application of uniaxial strain surprisingly yields two unreported, stabilized configurations with fully sp3hybridization: mC18 (C2 symmetry) and mP18 (P2/c symmetry). This phase transition, marked by an abrupt change in crystal volume and a profound alteration in electronic properties (transiting from a conductor to a semiconductor), bears the hallmarks of a first-order transition. The computational results highlight the exceptional properties of mC18, boasting a high density of 3.45 g/cm3 and a Vickers hardness of 91.41 GPa, while also exhibiting specific anisotropic elastic characteristics. Notably, structural and mechanical parallels are observed between mC18 and mP18, with their structural homology and equivalence further confirmed through topological analyses. Additionally, utilizing mC18 as a foundation, we have constructed a conductive two-dimensional graphyne, C24, and investigated its absorption properties under the extreme-condition of high-pressure. These findings demonstrate the promising potential applications of mC18 and its low-dimensional derivative, graphyne C24, in superhard materials and photoelectronic purposes.