Transition Metal Compounds (Rock Salt Structure)

Abstract

The interest in these groups of materials is related to the fact that many of them (the group V_b carbides and the group IV_b nitrides) are high-temperature superconductors. The general behavior of the phonon dispersion curves is still similar to those in other rock salt crystals and may well be described in terms of a shell model. Characteristic deviations appear in the optic modes at long wavelengths where the metallic screening destroys the Lyddane-Sachs-Teller splitting between the LO and TO modes, while at higher q values the Coulomb splitting between these modes can be clearly recognized. While this behavior is typical for all metallic crystals with optic modes, the phonon anomalies at q vectors with components q_α = 1/2 (in particular LA (1/2, 0, 0) and TA,LA at the L point (1/2, 1/2, 1/2) seem to correlate with high T_c’s in a way typical for this family of compounds. A successful model description was given by WEBER in his DSM [6.6, 13], while different microscopic explanations were given in [6.19, 25].