Along the vertical direction the frequency is given in units of Terahertz (THz). Since no negative energy magnitudes are observed, the three structures can be considered as dynamically stable. There are 3N-3 optical modes at high energy and 3 acoustic modes. The acoustic modes start from zero energy (ω = 0) at the Γ point, center of the Brillouin Zone, up to a few Terahertz. They correspond to the lattice rigid translation modes in the crystal (two transverse and one longitudinal). The remaining bands correspond to optic modes. Remarkably, in C 6 (Fig. ) the highest bands are at 50 THz, a magnitude higher than in diamond where ω max ~40 THz observed with Raman vibration spectroscopy. Such high frequency is assigned to the C2-C2 stretching. The phonons of Si 2 C 4 (Fig. ) have similar shape but the highest magnitude is at ω max ~45 THz. This is concomitant with the overall smaller elastic constants especially C 33 and the resulting B and G values as well as the hardness. The phonons dispersions of the equiatomic Si 2 C 2 N 2 show different features versus the two binaries with many flat bands resulting from the unique structure of SiC 2 N 2 tetrahedra and the strong C-N interaction.