The highest frequencies are observed for crb C 8 with bands around 40 THz, the signature of the tetrahedral carbon as in diamond . For BX phases, the lower frequencies are observed due to the trend of increasing d(B-X) along the series (cf. Table ). This is consistent with the trend of strong decrease in compactness with a change from broadband behavior for BN (Fig. ) to progressively flattened bands in BP (Fig. ), BAs (Fig. ), BSb (Fig. ), and finally BBi (Fig. ), which exhibits a dynamic instability characterized by the negative phonons at the  point. These trends are consistent with the decreasing cohesive energy along the series.