The energy level along the vertical line is with respect to the top of the valence band (VB), E V separated from the conduction band CB by an energy gap E gap of ~5 eV. Indeed, all three phases, isoelectronic with diamond, are wide band insulators. Particularly, C 16 shows the specific character of diamond of indirect band gap between  VB and Z CB (Fig. ). This feature also characterizes tetragonal C 6 N 8 (Fig. ), but B 8 N 8 (Fig. ) exhibits rather direct band gap  VB - CB .