Intrinsically Ultralow Thermal Conductivity in Ruddlesden-Popper 2D Perovskite Cs₂PbI₂Cl₂: Localized Anharmonic Vibrations and Dynamic Octahedral Distortions

Fundamental understanding of the correlation between chemical bonding and lattice dynamics in intrinsically low thermal conductive crystalline solids is important to thermoelectrics, thermal barrier coating, and more recently to photovoltaics. Two-dimensional (2D) layered halide perovskites have recently attracted widespread attention in optoelectronics and solar cells. Here, we discover intrinsically ultralow lattice thermal conductivity (κ_L) in the single crystal of all-inorganic layered Ruddlesden-Popper (RP) perovskite, Cs₂PbI₂Cl₂, synthesized by the Bridgman method. We have measured the anisotropic κ_L value of the Cs₂PbI₂Cl₂ single crystal and observed an ultralow κ_L value of ∼0.37-0.28 W/mK in the temperature range of 295-523 K when measured along the crystallographic c-axis. First-principles density functional theory (DFT) analysis of the phonon spectrum uncovers the presence of soft (frequency ∼18-55 cm^-1) optical phonon modes that constitute relatively flat bands due to localized vibrations of Cs and I atoms. A further low energy optical mode exists at ∼12 cm^-1 that originates from dynamic octahedral rotation around Pb caused by anharmonic vibration of Cl atoms induced by a 3s² lone pair. We provide experimental evidence for such low energy optical phonon modes with low-temperature heat capacity and temperature-dependent Raman spectroscopic measurements. The strong anharmonic coupling of the low energy optical modes with acoustic modes causes damping of heat carrying acoustic phonons to ultrasoft frequency (maximum ∼37 cm^-1). The combined effect of soft elastic layered structure, abundance of low energy optical phonons, and strong acoustic-optical phonon coupling results in an intrinsically ultralow κ_L value in the all-inorganic layered RP perovskite Cs₂PbI₂Cl₂.