Along the vertical direction the frequency is given in units of terahertz (THz). There are 3N-3 optical modes at high energy, and 3 acoustic modes. The acoustic modes start from zero energy (ω = 0) at the  point, center of the Brillouin zone, up to a few terahertz. They correspond to the lattice rigid translation modes of the crystal (two transverse and one longitudinal). While C 6 and B 2 C 2 N 2 present positive magnitudes of phonons, B 2 C 4 shows negative magnitudes at Z point (see blue arrow) pointing to very soft modes that provide it with instability. Note that the replacement of carbon by nitrogen provides a dynamic stability to the ternary phase.