LATTICE-DYNAMICS OF SKUTTERUDITES - FIRST-PRINCIPLES AND MODEL-CALCULATIONS FOR COSB3

The lattice dynamics of skutterudite structure CoSb3 are investigated by local-density frozen phonon calculations and force-constant model f its to both the results of these calculations and experimental frequen cies. First-principles calculations of the normal modes and frequencie s are reported for the Gamma-point A(g) Raman and A(u) phonons, as wel l as the constrained bulk modulus and its pressure derivatives. Cubic anharmonic terms are reported for the A(g) modes. Comparison of the re sults with an existing bond-stretching force-constant model reveals po or agreement for the lowest frequency A(u) phonon, whose displacement pattern is dominated by bond angle modulation with only small bond len gth changes. This lack of agreement is probably related to the highly covalent bonding of the material and the absence of bond angle force c onstants in the model. Improved force-constant models, which include b ond angle forces and reproduce both the first-principles and experimen tal phonon frequencies, were developed. Phonon-dispersion curves and d ensities of states are reported along with elastic constants and the D ebye temperature.