Thermal properties of CsPbCl3 thin films across phase transitions

The local thermal conductivity, thermal diffusivity, and volumetric heat capacity of cesium lead chloride perovskite thin films are mapped simultaneously and with highest spatial resolution by a scanning near-field thermal microscope. Both, the 3D phase (CsPbCl3) and the 0D phase (Cs4PbCl6) are investigated. For CsPbCl3 thin films the variation of the thermal properties across the phase transitions in the range from room-temperature to 65 degrees C are analyzed. While the thermal conductivity at room temperature is ultra-low, a significant increase of the thermal conductivity is found for the cubic phase of CsPbCl3 (T > 46 degrees C). While only slight variations in the thermal conductivity are detectable for transitions from the monoclinic to the orthorhombic to the tetragonal phase, thermal diffusivity and volumetric heat capacity measurements are extremely sensitive to the amount of heat involved in the respective transition. It is shown that upon transition to the cubic phase of CsPbCl3 thin films, the relative increase of the volumetric heat capacity is significantly higher than that of the thermal conductivity. Thus, the thermal diffusivity in the cubic phase becomes notably lower in comparison to that of the respective phase at room temperature. An increase of the volumetric heat capacity had been theoretically predicted earlier but could not be confirmed in previous experimental studies. The findings of our thermal analysis are of great general importance for fundamental material research and for the thermal design of thin-film devices based on CsPbCl3 perovskites.