Lattice dynamics of skutterudites: First-principles and model calculations forCoSb3

Abstract

The lattice dynamics of skutterudite structure ${\mathrm{CoSb}}_3$ are investigated by local-density frozen phonon calculations and force-constant model fits to both the results of these calculations and experimental frequencies. First-principles calculations of the normal modes and frequencies are reported for the Γ-point ${\mathit{A}}_{\mathit{g}}$ Raman and ${\mathit{A}}_{\mathit{u}}$ phonons, as well as the constrained bulk modulus and its pressure derivatives. Cubic anharmonic terms are reported for the ${\mathit{A}}_{\mathit{g}}$ modes. Comparison of the results with an existing bond-stretching force-constant model reveals poor agreement for the lowest frequency ${\mathit{A}}_{\mathit{u}}$ phonon, whose displacement pattern is dominated by bond angle modulation with only small bond length changes. This lack of agreement is probably related to the highly covalent bonding of the material and the absence of bond angle force constants in the model. Improved force-constant models, which include bond angle forces and reproduce both the first-principles and experimental phonon frequencies, were developed. Phonon-dispersion curves and densities of states are reported along with elastic constants and the Debye temperature.