Metallic Tetra-Penta-Hexa Graphene: A Stable Two-Dimensional Carbon Allotrope with High In-Plane Stiffness and Strain-DrivenStructural Transformation

Abstract

The discovery of novel two-dimensional (2D) carbon allotropes with tailored electronic and mechanical properties remains an important challenge in carbon materials research. Here, we predict a previously unexplored member of the tetra-penta-hexa (TPH) graphene family, termed TPH-M, through particle swarm optimization combined with first-principles calculations. The proposed monolayer is found to be energetically competitive within the family of carbon allotropes and satisfies the criteria for dynamical, mechanical, and thermal stability. Ab initio molecular dynamics simulations further demonstrate that TPH-M retains its structural integrity at elevated temperatures. In contrast to the previously reported semiconducting TPH phases, TPH-M exhibits an intrinsic nonmagnetic metallic electronic structure arising from delocalized C-p states crossing the Fermi level. The metallic state is preserved under moderate mechanical deformation, highlighting the robustness of its electronic properties. Mechanical calculations reveal pronounced elastic anisotropy together with an exceptionally high in-plane Young’s modulus comparable to the reported in-plane stiffness of graphene. Under large tensile strains, TPH-M exhibits distinct strain-dependent responses, including a structural transformation toward a biphenylene-like phase under uniaxial loading and significant modifications of its electronic structure under biaxial strain. The unique combination of intrinsic metallicity, thermal robustness, graphene-like mechanical rigidity, and strain-dependent structural and electronic responses establishes TPH-M as a noteworthy addition to the family of 2D carbon allotropes and provides new insights into the relationship between bonding configuration, mechanical deformation, and electronic properties in low-dimensional carbon systems.