Accurate Measurement of Thermal Displacement in Filled Skutterudite by Scanning Transmission Electron Microscopy

The efficiency of thermoelectric materials is characterized by the dimensionless thermoelectric figure of merit, ZT = T/ρκ, where  is the Seebeck coefficient, ρ the electrical resistivity, T the temperature, and к the total thermal conductivity. To achieve high ZT, one needs to maximize the power factor /ρ, whereas minimize thermal conductivity к. The unfilled skutterudites have a high power factor, but their thermal conductivity is moderately low, thus ZT is small. High ZT can be achieved in filled skutterudites by filling guest atoms into the cages formed by pnicogen atoms [1]. The filling atoms are loosely bound to the pnicogen atoms, leading to Einstein-like vibrational modes that significantly reduce the lattice thermal conductivity. Measurement of local disorder and lattice vibrations, especially thermal vibrations of the filled atoms is therefore important for understanding the underlying mechanisms of the reduced thermal conductivity in the filled skutterudites. Recently, we showed that thermal and static displacement in the layered thermoelectric Ca3Co4O9 can be determined accurately by simultaneous acquisition of high-angle-and mediate-angleannular dark field images in scanning transmission electron microscopy (STEM) [2]. Here, we demonstrate that the thermal displacement in filled skutterudite can be measured by acquiring STEM images with annular dark field (ADF) and annular bright field (ABF) detectors simultaneously (Fig. 1a). Unlike diffraction analysis that derives the overall displacements in the crystal from the intensities of Bragg reflections, we directly measure the atomic displacement in real space, thereby enabling us to refine independently the atomic displacement of the filled atoms that is crucial to revealing their different nature in phonon scattering.