Cage-C22 and Cage-C28: Two novel superhard orthorhombic carbon allotropes

Abstract

In this work, two novel three-dimensional Cage carbon allotropes with all-sp3 and mixed sp2-sp3 hybridized networks, namely, Cage-C22 and Cage-C28 are theoretically predicted by the analysis executed using based on the density functional theory. Cage carbon superhard crystals possess Pmmm (47) symmetry (space group no. 25) which are orthorhombic crystals. The electronic properties, mechanical properties (hardness, elastic anisotropy, elastic modulus, and directional dependence of Young's modulus in GPa), and structural properties for both the carbon phases are systematically investigated and analyzed. The calculated phonon dispersions and elastic coefficients Cij show that Cage carbon allotropes are dynamically and mechanically stable at ambient conditions (0 GPa). To study the mechanical properties of these structures, the (Young's, shear, bulk) modulus were obtained by using the Voigt-Reuss-Hill approximation. The ratios of Emax/Emin as significant ratios of mechanical anisotropy in Young's modulus are greater than that of diamond (1.10) which means that Cage structures show elastic anisotropy. Furthermore, the electronic properties of these two new structures show that Cage-C22 crystal is a direct semiconductor and its gap energy is about 1.5 eV at Z point, while Cage-C28 is a metal because it has zero band gap between valence bands (VB) and conduction bands (CB). The calculated X-ray spectrum patterns show noncrystalline behavior which is derived from its structural complexity, and bond angles and bond lengths are also effective in this matter. The calculated results indicate that these two 3D superhard structures have the required potential for the use of cathode materials in capacitors, electrical and mechanical applications.