Thermal Conductivity Reduction by Fluctuation of the Filling Fraction in Filled Cobalt Antimonide Skutterudite Thermoelectrics

The reduction of the lattice thermal conductivity is one of the crucial steps in improving thermoelectric materials. In skutterudites, a well-known approach is to reduce the thermal conductivity by filling the structural cage with rare-earth atoms. In this work, we show that it is not just the amount of such filling itself but its nanoscale structuration that lowers the thermal conductivity. A straightforward synthesis procedure under high pressure yields Ce- and Yb-filled CoSb3 skutterudites, with and without an inhomogeneous distribution of the filler atoms. The composition of the phases is evaluated from synchrotron X-ray diffraction (SXRD) data; the highly nanostructured morphology is verified by high-resolution transmission electron microscopy (TEM). The filling fluctuation, i.e., the uneven distribution of filling atoms in the sample originating a phase segregation, brings about low lattice thermal conductivity, as a strong source of phonon scattering. This effect is prominent in the Ce-filled compo...