As mentioned above, the archetypical crb C 8 carbon allotrope belongs to body-centered tetragonal ("I" centering: from German Innenzentriert) space group I4/mmm (No. 139) with a unique carbon eight-fold Wyckoff position (Table ). This configuration does not permit the occupation of two chemically distinct elements to form a binary compound. From crystal chemistry engineering, crb C 8 was redefined in a lower symmetry primitive "P" space group, specifically P4 2 /mnm (No. 136), with two distinct four-fold carbon positions. These positions are described in Table and shown in two different colors (brown and white) in Fig. . In both configurations the C-C distances are 1.519 Å and 1.574 Å (square). These values are on either side of the sum of the atomic radii of two carbon atoms with r(C) = 0.77 Å, which is 1.54 Å, i.e. the C-C distance in diamond, the perfectly covalent crystal structure. The cell volume of 47.75 Å 3 is the same in both "I" C 8 and "P" (C1) 4 (C2) 4 , resulting in a density ρ=3.34 g/cm 3 lower than that of diamond: ρ = 3.50 g/cm 3 suggesting a slightly lower hardness of both crb allotropes. The peculiarity of the structures with crb topology is the presence of a C4 square with a perfect angle of 90° and another angle of ∠C-C-C1 = 113.62°. The building block tetrahedra are distorted, thus showing a clear difference with respect to diamond, where a unique ideal tetrahedral angle of 109.47° is identified.