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

Abstract

AbstractAs 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.