The band structures include 3N bands: three acoustic modes starting from zero energy (ω = 0) at the  point, the center of the Brillouin zone, up to a few terahertz, and 3N-3 optical modes at higher energies. The acoustic modes correspond to the lattice rigid translation modes of the crystal (two transverse and one longitudinal). All six panels in Fig. show positive phonon frequencies, indicating the dynamic stability of all investigated phases. This observation is significant because the crb BX phases become less cohesive along the series (Table ).