d-band effects in the lattice dynamics of transition metal carbides

We present the first ab initio. calculation of the d-bandstructure contribution to the acoustic phonon frequencies of NbC. The results, obtained. by freezing in the lattice vibrations within the rigid muffin tin approximation, give anomalies in the longitudinal modes in agreement with experiment. The position of the 〈001〉 and 〈111〉 soft modes are sensitive to the band filling. The transverse anomalies are not explained by pure d-interactions, highlighting the importance of p-d hybridization in the carbides.