A novel crystallographic location of rattling atoms in filled Eu x Co4Sb12 skutterudites prepared under high-pressure conditions

Abstract Thermoelectric M x Co 4 Sb 12 skutterudites are well-known to exhibit a reduced thermal conductivity thanks to the rattling effect of the M-filler at the large cages occurring in the framework, centered at the 2 a sites of the I m 3 ‾ $Im\overline{3}$ space group. A novel Eu-filled skutterudite has been synthesized under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. The structural refinement from high-angular resolution synchrotron X-ray diffraction (SXRD) patterns unveils an unusual position for Eu filler atoms. By difference Fourier synthesis they are found at 12 d sites, conforming statistically occupied octahedra within the mentioned cages around 2 a positions. The Debye temperature was estimated by averaging the isotropic displacements by the atomic masses, leading to θ D ${\theta }_{D}$ of 273(2) K. Oftedal plots concerning the y and z Sb fractional positions, the unit-cell parameter a and M filling fraction include the novel Eu specimen in the trend observed for other filled materials prepared under high-pressure, including rare-earths, alkali or alkali-earth elements, all accepted as rattlers in filled skutterudites. A total thermal conductivity ( κ ) of 0.82 W m −1 K −1 is measured at 773 K for Eu 0.02(1) Co 4 Sb 12 , below that of other filled skutterudites, which is promoted by the enhanced phonon scattering of Eu located at 12 d sites. FE-SEM images showed large, homogeneous grains, well compacted after the high-pressure synthesis.