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 in 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 (kL) in the single crystal of all-inorganic layered Ruddlesden-Popper (RP) perovskite, Cs2PbI2Cl2, synthesized by Bridgman method. We have measured anisotropic kL of Cs2PbI2Cl2 single crystal and observed ultralow kL 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 originating from dynamic octahedral rotation around Pb caused by anharmonic vibration of Cl atoms induced by 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 cause damping of heat carrying acoustic phonons to ultra-soft 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 intrinsically ultralow kL in the all-inorganic layered RP perovskite Cs2PbI2Cl2.

Intrinsically ultralow thermal conductivity in Ruddlesden-Popper 2D perovskite Cs2PbI2Cl2: Localized anharmonic vibrations and dynamic octahedral distortions

ACS Spring 2022

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