The 9R diamond polytype (hRC 18 ; space group R-3m No. 166) is shown in Fig. , in two representations: ball and stick (left) and tetrahedral stacking (right). C4 tetrahedra are corner sharing within 9 layers like 9-layered SiC polytype. Table The introduction of a trigonal sp 2 carbon was achieved by inserting of a 4 th carbon at the (3a) 0,0,0 site resulting in a C 21 stoichiometry. The resulting constrained structure with short C-C distances was relaxed along successive cycles of geometry optimization until reaching the ground state configuration whose parameters are shown in Table , 2 nd column, characterized by the shortest distance: 1.46 Å assigned to the C=C bond, larger by 0.11 Å than that in the ethylene molecule (~1.35 Å). The perturbation of the 9R diamond polytype (hRC 18 ) leads to a decrease of the cohesive energy E coh = 2.061 eV/atom. The structure shown in Fig. exhibits a different stacking of tetrahedra and a less compact structure quantified by the decrease in density from ρ = 3.53 g/cm 3 in C 18 to 3.34 g/cm 3 in C 21 .