  The electron localization tensor, Born effective charges, dielectric
constants and phonon dispersion relations of the skutterudite CoSb$_3$ and the
filled skutterudite TlFeCo$_3$Sb$_{12}$ have been studied using density
functional perturbation theory. The origin of the low energy peak in the phonon
density of states of TlFeCo$_3$Sb$_{12}$ observed recently by Neutron inelastic
scattering [R. P. Hermann {\it et al.}, Phys. Rev. Lett. {\bf 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.
