Stability, electronic and mechanical properties of superhard materials formed by 4+6+8 membered rings of carbon

Abstract

In order to study the relationship between structures and properties, a series of 4 ​+ ​6+8 carbon allotropes including oC24, oC32, oC40 and oC48 are designed by inserting eight-atom fragments between the four- and eight-membered rings of bct-Carbon and oC16-Ⅱ carbon. The calculations prove that these carbon phases are thermodynamically, mechanically and dynamically stable and the new designed oC40 and oC48 possess a higher bulk module, shear module, Young’s modulus and Vickers hardness than the theoretically proposed o-C32 and M-carbon. The calculated relative enthalpies against pressure suggest that oC40 and oC48 can be transformed from graphite under a lower pressure than bct-Carbon and o-C16-Ⅱ. Thus oC40 and oC48 are expected to be experimentally synthesized from cold-compressed graphite. More importantly, the calculated results show that the stability, bandgaps, bulk modules, shear modules, Young’s modulus, incompressibility, ideal tensile strengths and Vickers hardness of these structurally similar allotropes increase with increasing proportion of the six-membered rings of carbon, suggesting that their stability and properties can be achieved by engineering their microstructures.