First-principles investigation of the structural, elastic, anisotropic and electronic properties of Pmma-carbon

An investigation of the structural, elastic, anisotropic and electronic properties of the recently reported Pmma-carbon was performed by means of first-principles calculations in this paper. The Pmma-carbon phase possesses an orthorhombic symmetry with space group Pmma (No. 51). It contains 10 carbon atoms per unit cell and adopts an all-sp3 4+6+8-membered rings bonding network. The structural stability of Pmma-carbon is verified by analysis of the total energy, phonon spectra and elastic constants. The obtained bulk modulus and shear modulus indicate that Pmma-carbon is a brittle material and possesses elastically anisotropic. It is identified that Pmma-carbon is a potential superhard material with a Vickers hardness surpassing 70 GPa, which is lower than that of diamond but comparable with c-BN. The analysis of electronic band structure and density of states reveals that Pmma-carbon is an insulator with a quasi-direct band gap of 4.45 eV. The X-ray diffraction patterns of Pmma-carbon are also provided for future identifying the carbon phase in experiments. The present findings enrich the existing superhard carbon materials family and broaden our understanding of superhard carbon phases.