Ultralow thermal conductivity in 1D and 2D imidazolium-based lead halide perovskites

Low-dimensional hybrid organic-inorganic metal halide perovskites are rapidly emerging as a fascinating sub-class of the three-dimensionalparent structures, thanks to their appealing charge and thermal transport properties, paired to better chemical and thermal stabilities.Extensive investigations of the thermal behavior in these systems are of paramount relevance to understand their optoelectronic and thermoelectricapplications. Herein, we present a complete thermophysical characterization of imidazolium lead iodide, (IMI)PbI3, a 1D pseudoperovskitewith chains of face-sharing octahedra, and histammonium lead iodide, (HIST)PbI4, a 2D layered perovskite with corner-sharingoctahedra. Upon heating, the two compounds show highly anisotropic thermal expansion effects and high thermal stability until 250-300 C.The thermal diffusivity of pelletized powders was measured with the laser flash technique from room temperature up to 225 C. To accountfor the reduced density of the pelletized powders with respect to the bulk, the diffusivity data in different atmospheres were modeled as afunction of the volume fraction and dimensionality of the pores, allowing to extrapolate the thermal conductivity of the bulk materials. Thetwo compounds exhibit an ultralow thermal conductivity of 0.15W/mK, two to three times lower than that reported on 3D MAPbI3 usingthe same technique. This finding suggests the primary role of the organic molecules within the hybrid systems, regardless of the octahedraconnectivity and dimensionality.