Penta-C₂₀: A Superhard Direct Band Gap Carbon Allotrope Composed of Carbon Pentagon

A metastable sp³-bonded carbon allotrope, Penta-C₂₀, consisting entirely of carbon pentagons linked through bridge-like bonds, was proposed and studied in this work for the first time. Its structure, stability, and electronic and mechanical properties were investigated based on first-principles calculations. Penta-C₂₀ is thermodynamically and mechanically stable, with equilibrium total energy of 0.718 and 0.184 eV/atom lower than those of the synthesized T-carbon and supercubane, respectively. Penta-C₂₀ can also maintain dynamic stability under a high pressure of 100 GPa. Ab initio molecular dynamics (AIMD) simulations indicates that this new carbon allotrope can maintain thermal stability at 800 K. Its Young's modulus exhibits mechanical anisotropy. The calculated ideal tensile and shear strengths confirmed that Penta-C₂₀ is a superhard material with a promising application prospect. Furthermore, Penta-C₂₀ is a direct band gap carbon based semiconducting material with band gap of 2.89 eV.