Atomic-Scale Observation of Off-Centering Rattlers in Filled Skutterudites

As one of the most fundamental order parameters, the coordinate order of atoms is a cornerstone in determining the physical and chemical properties of materials. Off-centering shifts of atoms from their centrosymmetric positions, a phenomenon that is closely related to symmetry breaking and structural transitions in crystals, is of key importance in modifying a broad range of fundamental properties, such as ferroelectricity, antiferroelectricity, and piezoelectricity. In thermoelectric materials, which can be used to directly interconvert heat to electricity, off-centering has been proposed in, e.g., lead tellurides, halide perovskites, type I and II clathrates, and filled skutterudites, to be the physical origin of the experimentally measured exceptionally low thermal conductivity, but only indirectly supported from structural refinements and experimental/theoretical vibration mode analyses. In this work, Yb partially filled and Ce fully filled skutterudite is taken as a model system for directly measuring the off-centering shifts of filler atoms, i.e., rattlers, with picometer precision in real space, by means of atomic-resolution negative spherical aberration imaging transmission electron microscopy. This finding allows an in-depth understanding of the relationship between the off-centering and the thermoelectric performance, and the presented methodology here is also applicable to investigate the off-centering phenomena in other functional materials, e.g., ferroelectrics and solar cells.