Recent advances in filled skutterudite systems

A lot of the studies on filled skutterudites so far have focused on searching for new filled compounds and revealing the influence of filling Sb-dodecahedron cage on the thermoelectric performance. For a given host skutterudite system the thermodynamic stability of filled skutterudite structure and therefore the filling fraction limit are greatly influenced by the ionic radius and the valence state of the filler elements. As for Co₄Sb₁₂-based filled skutterudite system, the stable filled compounds reported so far cover those in which the ratio of radii of filler ion and the Sb-dodecahedron cage (r_ion/r_cage) ranges from 0.6 to 0.9. The filling fraction limit increases with increasing ionic radius and with decreasing ionic valence of the fillers. The "rattling" behavior of filler atoms has been well characterized by thermal parameters. A smaller ion shows a larger thermal parameter and causes a greater reduction of lattice thermal conductivity. For a given host skutterudite structure, the lattice thermal conductivity decreases with filling fraction and reaches a minimum at the filling fraction of about 30-40%. Multi-filling is an effective approach to generate additional phonon modes in the filled skutterudite structure and further depress lattice thermal conductivity. As for the multi-filling approach, the stability of the filled skutterudite structure depends strongly on the difference of the chemical and physical characteristics such as ionic radius and electronegativity between the co-filler elements. Further optimization of the material system for multi-filled skutterudites is expected to lead to further improvements in thermoelectric performance.