Turning to the ternary BCN compound, since two C2 belong to the same Wyckoff site (4i) the replacement of one carbon by nitrogen leads to B 2 C 2 N 2 (or BCN) where B is tetrahedral (sp 3 -like), and N and C occupy the C trigonal sites. The initial tetragonal space group P4 2 /mmc (N°131) symmetry is consequently lowered, and subsequent crystal structure determinations after geometry relaxation led to define the ternary phase in a new space group P4 2 mc (N°105) (Fig. ), i.e. with the loss of a mirror (m) symmetry due to the occupation of the fourfold (4i) ½, 0, z position in B 2 C 4 (Table ) by two different atoms, N and C, which are now are in twofold (2c) ½, 0, z positions (Table ). In C 6 (Fig. ) the charge density around C1 tetrahedral site shows sp 3