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7225518 
Journal Article 
Intrinsically Ultralow Thermal Conductivity in Ruddlesden-Popper 2D Perovskite Cs2PbI2Cl2: Localized Anharmonic Vibrations and Dynamic Octahedral Distortions 
Acharyya, P; Ghosh, T; Pal, K; Kundu, K; Rana, KS; Pandey, J; Soni, A; Waghmare, U; Biswas, K; , 
2020 
Yes 
Journal of the American Chemical Society
ISSN: 0002-7863
EISSN: 1520-5126 
AMER CHEMICAL SOC 
WASHINGTON 
15595-15603 
32799442 
WOS:000571437600044 
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 (kappa(L)) in the single crystal of all-inorganic layered Ruddlesden-Popper (RP) perovskite, Cs2PbI2Cl2, synthesized by the Bridgman method. We have measured the anisotropic kappa(L) value of the Cs2PbI2Cl2 single crystal and observed an ultralow kappa(L) value of similar to 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 similar to 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 similar to 12 cm(-1) that originates from dynamic octahedral rotation around Pb caused by anharmonic vibration of Cl atoms induced by a 3s(2) 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 similar to 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.L value in the all-inorganic layered RP perovskite Cs2PbI2Cl2.