tetrahedral C1 and trigonal C2, the latter forming C2-C2 pairs that separate the C1C2 4 tetrahedra. C 6 is consequently expressed as C1 2 C2 4 . The replacement of C1 by B leads to hypothetic boron dicarbide B 2 C 4 (Fig. ) which shows the same tetrahedral stacking as in C 6 .