Dual Crystal-Liquid Thermal Transport Behavior in MAPbCl.

Phonon dynamics in organic-inorganic hybrid perovskites (OIHPs) exhibit inherent complexity driven by the intricate interactions between rotatable organic cations and dynamically disordered inorganic octahedra, mediated by hydrogen bonding. This study aims to address this complexity by investigating the thermal transport behavior of MAPbCl as a gateway to the OIHPs family. The results reveal that the ultralow thermal conductivity of MAPbCl arises from a synergistic interplay of exceptionally low phonon velocities, short phonon lifetimes, and phonon mean free paths approaching the Regel-Ioffe limit. Additionally, the thermal conductivity of MAPbCl approaches its theoretical amorphous limit across a broad temperature range, with its thermal transport behavior transitioning from crystal-like to more liquid-like during the orthorhombic-to-cubic phase transitions. Furthermore, a phonon drag effect is observed at 17 K, with Umklapp scattering serving as the predominant phonon resistive mechanism in the orthorhombic phase. In contrast, dynamic lattice distortions caused by the jumping rotation of MA cations become the primary factors influencing thermal transport in the cubic phase.