A possible superhard orthorhombic carbon

• An orthorhombic superhard carbon allotrope is predicated by Particle Swarm Optimization method.
• Investigations show that it is an indirect band gap crystal and stable at 0 GPa.
• Computed hardness and ideal strength beyond 69.0 GPa indicate its superhard character.
• Related microscopic deformation mechanisms under tension and shear are detailedly analyzed.

Carbon is a versatile element in the periodic table because of its ability to form various stable and metastable allotropes. In the present work, an orthorhombic carbon allotrope with ‘4-membered rectangles’ is predicated by Particle Swarm Optimization method at 0 GPa. Calculations indicate that this new phase is dynamically and mechanically stable although it has a high enthalpy. Its electronic and mechanical properties such as elastic constants, hardness, ideal tensile strength and shear strength are systematically studied. The results show that it is an indirect band gap crystal with a gap of 2.68 eV; in spite of its elastic anisotropy, the theoretical hardness of 76.2 GPa still makes it a potential superhard material; the obtained ideal tensile strength and shear strength are 69.0 GPa and 75.6 GPa, respectively, confirming its superhard character. The related microscopic deformation mechanisms are also detailedly analyzed through investigating the corresponding charge density, which reveals that the bonds arranged parallel to the b-axis are responsible for the breakdown of the crystal under the critical tension and shear deformations. These studies provide important information for the potential applications of the crystal once it is synthesized.