Both structures are defined in tetragonal space group P4 2 /mmc, N° 131 (cf. Tables &). Since the two C2 belong to the same Wyckoff site (4i), the replacement of one carbon by nitrogen leads to Si 2 C 2 N 2 where Si is tetrahedrally coordinated with trigonal N and C. The initial tetragonal space group P4 2 /mmc N° 131 symmetry is 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 Si 2 C 4 (Table ) by two different atoms, N and C, which are now are in twofold (2c) ½, 0, z positions (Table ). The trigonal carbon interatomic distances increase from C 6 with d(C2-C2) =