In the case of the tetragonal phases, besides the dispersed bands, the flat bands at ~39 THz for B 3 N 2 (Fig. ) and the higher frequency band at ~58 THz for B 3 N 3 (Fig. ) are observed. The 39-THz band for tetragonal B 2 N 3 can be attributed to the B-N distance within the tetrahedron, while the 58-THz band for tetragonal B 3 N 3 can be assigned to the B-N distance in the B-N-B fragment along the c-axis. For hexagonal phases such flat bands are absent while observed bands with frequencies of about 40 THz are due to the larger B-N distances in these phases (see Table ).