Dense as diamond: Pn-C₁₀, a superhard sp³ carbon allotrope

We introduce a carbon allotrope with rhombic symmetry and a high crystal density reached 3.56 g/cm³, named Pn-C₁₀, which was found using an unbiased particle-swarm structural-searching technique. This fully sp³-bonded carbon phase is dynamically and mechanically stable in its ground state. Results from our calculations reveal the excellent mechanical nature of Pn-C₁₀ with a claimed shear modulus and Vickers hardness of 484 GPa and 88.45 GPa, respectively, which also suggested its slight anisotropy in elasticity. The predicted electronic band structure indicates that Pn-C₁₀ is an indirect-bandgap material with a bandgap of 5.05 eV. Natural tiling analysis and simulations of x-ray diffraction and Raman spectra were performed to provide insights for further studies into Pn-C₁₀ and other carbon phases.