Skutterudites are promising candidates for effective thermoelectrics due to their cage-like crystal structure. Guest atoms can fill large voids of the cubic crystal structure (2a crystallographic site) (Fig. 1). These are only weakly bound to the host structure and interact with the lattice vibrations lowering the lattice thermal conductivity and thus increasing the figure of merit ZT [1-3]. Further, guest atoms affect the charge carrier density leading to a change in the electrical properties and ZT as well. Thus, among other aspects, the knowledge of the actual filling fraction of the guest atom on the crystallographic 2a site is of great importance for a complete understanding of the thermoelectric properties.
Fig. 1: Crystal structure of rare-earth R-filled CoSb3 skutterudite, RCo4Sb12. Co atoms (blue) and Sb atoms (red) form the host structure while the rare-earth filler atoms (orange) occupy a fraction of the voids on the 2a site (here for clarity shown with full occupancy). The coordination of the filler atom in the large Sb-cage is indicated by the light orange icosahedron.
In one of our current projects carried out in collaboration with the Institute of Materials Research at DLR, Cologne, we investigate RxCo4Sb12 skutterudites filled with a rare-earth element R. The material properties strongly depend on the actual filling fraction x, as the thermal conductivity is expected to decrease with increasing concentration of filler atoms at the 2a site. Simultaneously, the filler acts as a dopant, affecting both the electrical conductivity and the Seebeck coefficient. Furthermore, the formation of secondary phases, an issue often encountered in the synthesis of skutterudites, is relevant for the material performance, as secondary phases usually deteriorate the thermoelectric properties. Finally, adding to the complexity of the optimization challenge, the specific type of secondary phase and its weight fraction may depend on details of the synthesis route.
We employ neutron powder diffraction which offers the opportunity to determine the filling fraction x of the 2a guest site from Rietveld refinements. Moreover, information on structural defects and the type and amount of secondary phases can be obtained as well. Combining the microscopic structural information with the macroscopic transport properties thus allows to gain a detailed understanding of the behaviour of the thermoelectric properties with varying nominal degree of filling and to provide further guidance on optimized synthesis strategies to achieve higher ZT.
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 G.S. Nolas, G.A. Slack, T. Caillat, G.P. Meisner, Raman scattering study of antimony‐based skutterudites, J. Appl. Phys. 79 (1996) 2622-2626.
 G.S. Nolas, G.A. Slack, D.T. Morelli, T.M. Tritt, A.C. Ehrlich, The effect of rare‐earth filling on the lattice thermal conductivity of skutterudites, J. Appl. Phys. 79, (1996) 4002-4008.