In this context, we considered the reduction of the sp 2 /sp 3 extended stoichiometries allotropes, and C 10 , C 14 and C 18 were proposed as derived respectively from 4H, 6H and 8H diamond polytypes, respectively, and also characterized by ultrahardness close to diamond, and semi-metallic electronic structure due to the presence of C(sp 2 ) . The purpose of the present work was to study the changes in the mechanical and electronic properties of cfe 9R (hRC 18 ) allotrope, referred to as C 18 in the present work, by the insertion of sp 2 and sp 1 carbon atoms in two stoichiometries, based on crystal chemistry rationale and subsequent first-principles ground state energy calculations of stable structures and derived quantities within the well-established quantum mechanics framework of the density functional theory DFT . The two novel allotropes: ene-C 21 with sp 3 -sp 2 mixed hybridizations and yne-C 24 with sp 3 -sp 1 mixed hybridizations retain the same cfe topology of the original C 18 . ene-C 21 and yne-C 24 were found unique from the CCDC curation and provided with corresponding DOI references .