Filled skutterudites have become a topic of considerable interest with respect to basic and applied solid state sciences. This is particularly due to a variety of physical properties which can be intimately related to the underlying structural chemistry. Their stoichiometry can be rationalized with the chemical formula MT₄X₁₂, with M being an electropositive element like alkali, alkaline-earth, raere-earth, actinide or thallium metal, T standing for a transition metal of the iron- or cobalt-group and X representing a pnicogen element as they are phosphorus, arsenic or antimony. They all crystallize with the cubic LaFe₄P₁₂ structure where La stabilizes the iron-phosphorus host structure and is located inside an icosahedral cage formed by phosphorus atoms.

Figure 1. Magnetic susceptibility χ(T) of filled skutterudites MPt4Ge12 (M= Sr, Ba, La, Pr) for μ₀H = 2 mT (nominally). The inset shows the inverse molar susceptibility H/M of the Pr compound for μ₀H = 1 T.
R. Gumeniuk, W. Schnelle, H. Rosner, M. Nicklas, A. Leithe-Jasper, Yu. Grin, Phs. Rev. Lett. 100 (2008) 017002

Very recently it has been shown that the transition metal is not restricted to the iron or cobalt group but can also be the noble metal platinum together with germanium as the frame-work forming element. LaPt₄Ge₁₂ and PrPt₄Ge₁₂ (with a crystal electric field singlet ground state) are superconductors below 8.3 K. 

Electronic structure calculations and chemical bonding analysis by the electron localizability indicator (ELI) describe the MPt₄Ge₁₂ compounds as covalently bonded polyanionic [Pt₄Ge₁₂] framework structures which are stabilized by electron transfer from the filler metal-ions M.

Figure 2. Electron localizability indicator in LaPt₄Ge₁₂. The isosurface of ELI-D reveals the structuring of the penultimate shell of Pt (non-spherical distribution is here visualized around the pink sphere which denotes the position of the Pt nucleus) and appearance of the maxima on the Ge-Ge bonds. The maxima corresponding to the Pt-Ge bonds occur at a lower ELI-D value and therefore are not shown. The bond counts are obtained from the integration of the electron density within the basins of the ELI attractors.
Gumeniuk R., Borrmann H., Ormeci A., Rosner H., Schnelle W., Nicklas M., Grin Yu., Leithe-Jasper A. Z. Kristallogr. 225 (2010) 531.

The new ternary samarium-filled platinum –germanium skutterudite SmPt₄Ge₁₂ was prepared at a pressure of 5 GPa and a temperature of 1070 K. X-ray absorption spectroscopy measurements show that samarium in SmPt₄Ge₁₂ has a temperature-independent intermediate valence (ν = 2.90±0.03).

Figure 3. X-ray absorption spectra (XAS) on the Sm L_III edge: SmPt₄Ge₁₂ at various temperatures (symbols) and Sm₂O₃ at T = 300 K (solid line).
Gumeniuk R., Schöneich M., Leithe-Jasper A., Schnelle W., Nicklas M., Rosner H., Ormeci A., Burkhardt U., Schmidt M., Schwarz U., Ruck M., Grin Yu. New J. Phys. 12 (2010) 103035.

The low-temperature specific heat displays a broad anomaly centred at 2.9 K and a large linear coefficient γ'= 450 mJ mol^-1 K^-2 suggesting heavy-fermion behaviour. Low-temperature electrical resistivity shows temperature dependence reminiscent of the Kondo effect.

Moreover, skutterudites can be also stabilized by tin as can be seen for Sn_xPt₄Sn_ySb_12-y, a compound which is the first representative of the filled skutterudite family with the filler atoms (Sn) covalently bonded to the cavity's wall.

Figure 4. Sn atoms occupying the icosahedral voids in Sn_xPt₄Sn_ySb_12-y with a shift from the center of the cage.
Y. Liang, H. Borrmann, M. Baenitz, W. Schnelle, S. Budnyk, J. T. Zhao, Yu. Grin, Inorg. Chem. 47 (2008) 9489-9496

Furthermore, possible applications in the field of thermoelectrics moved the family of filled skutterudites into the focus of materials engineering.

The crystal structure ( ref.1,  ref.2) / electronic structure ( ref.1,  ref.2) as well as the  chemical physics of skutterudites is explored at the MPI-CPfS.