Exceptional ring topology makes diamond allotropes as light-weight superhard materials

Abstract

Ring topology (RT) is defined as minimal closed rings to characterize the connection features of given atoms so that RT can identify beyond the nearest neighbors the structures of allotropes bearing the same local bonding features, e.g. four-fold coordination in diamond-related materials. Two diamond allotropes with less common 5–8 membered ring topologies were studied by the first-principles calculations in this work. These orthorhombic carbon structures, denoted as L-PHOD and Z-PHOD carbons, respectively, are three dimensional networks connected solely via sp3 hybridized CC bonds but their atoms are arranged in a non-hexagonal ring topology different from the conventional diamond. They can be constructed by superimposing Octagon-Pentagon Graphene monolayer consisting of (5–8)-membered rings connected along a straight or a zigzag path. The L-PHOD and Z-PHOD carbons are predicted to be semiconductors with indirect band gaps ~4–5eV. Compared with diamond the postulated L-PHOD and Z-PHOD carbons are ~20% softer in hardness and weaker in tensile strength but ~10% lighter in mass densities due to their larger internal channels associated with the octagon rings, implying their potential applications as light-weight superhard materials. Our work suggest that tuning the ring topology of diamond-related materials provides a design strategy to balance the mechanical properties and densities often required in the development of light-weight structure materials.