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

A B S T R A C T

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.