Geometric configuration of polyhedrons dominating particle- to wave-like thermal transport in single crystalline Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub>

The thermal transport in a lattice is governed by the structural chemistry, which is critically important for heat flux management in thermoelectric conversion. Here, we reveal the structural chemistry origin that determines the thermal transport behavior and the marked anisotropy in single-crystalline Cs3Bi2I9 . Along the crystallographic c-axis direction, the particle-behaviordominated thermal conductivity (k) is approximately 0.3 W/m-K at room temperature and decreases with increasing temperature.In contrast, the thermal transport perpendicular to the c-direction is primarily wave-like behavior, exhibiting a room-temperature k of 0.17 W/m-K that increases at elevated temperatures. This fundamental difference in thermal transport mechanisms arises from distinct chemical bonding environments, specifically the strong binding force between the face-sharing [BiI6] octahedrons parallel to the c-direction and the weak rattling behavior between corner-sharing [CsI12] cuboctahedrons perpendicular to the c-direction. These findings provide a design principle for manipulating thermal transport through crystal chemistry engineering, opening avenues for developing anisotropic thermoelectric materials with thermal management capabilities.