Lattice dynamics of filled skutterudites:La(Fe,Co)4Sb12

Abstract

Materials for thermoelectric application should have low lattice thermal conductivities. Here we use a comparison of theoretical calculations and Raman spectroscopy to study the lattice dynamics of a La filled skutterudite and identify the Raman active modes. Polarized Raman spectra from polycrystalline ${\mathrm{La}}_{0.75}{\mathrm{Fe}}_{3}{\mathrm{CoSb}}_{12}$ are presented. First principles calculations of atomic forces generated by small displacements from the ideal skutterudite structure are utilized in a least squares procedure to evaluate the general force constant tensors. These are used to predict the normal mode frequencies and displacement patterns. The relative Raman intensity for each vibration is then estimated using a bond polarizability model. We find that both the centers of gravity of the eight symmetry-allowed Raman peaks and their relative intensities are in excellent agreement with the theoretical predictions. This agreement supports our mode assignments, particularly for those of ${A}_{g}$ symmetry, which exhibit a strong polarization dependence. The ``rattling'' vibrations of interstitial La are expected to be Raman inactive for an ideal fully filled material and are not observed.