Interestingly, the fracture toughness: K Ic (C 18 ) = 6.5 MPa•m ½ is greater than K Ic (lonsdaleite) = 6.2 MPa•m ½ , to the best of our knowledge, no mechanical properties for this polytype are available in the literature. The two hybrid allotropes show higher toughness for ene-C 21 . The trends of higher performance of C 18 versus diamond and lonsdaleite are also observed for Young's modulus (E) and