First-principles study of filled and unfilled skutterudite antimonies

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The electron localization tensor, Born effective charges, dielectric constants and phonon dispersion relations of the skutterudite CoSb₃ and the filled skutterudite TlFeCo₃Sb₁₂ have been studied using density functional perturbation theory. The origin of the low energy peak in the phonon density of states of TlFeCo₃Sb₁₂ observed recently by Neutron inelastic scattering [R. P. Hermann et al., Phys. Rev. Lett. 90, 135505 (2003)] is attributed to the vibrations of Tl that show only weak coupling with the normal modes of the host crystal. Moreover, the dielectric properties of these materials show unsual features such as giant Born effective charges and a strong increase of the optical and static dielectric tensor in the filled compound.