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

Abstract

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, Cs 2 PbI 2 Cl 2 , synthesized by the Bridgman method. We have measured the anisotropic κ L value of the Cs 2 PbI 2 Cl 2 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 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 ∼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 Cs 2 PbI 2 Cl 2 .

Slowing down the heat in thermoelectrics

“…Cs 2 PbI 2 Cl 2 , a 2D layered Ruddlesden−Popper perovskite employed in photovoltaics and optoelectronics, has recently been reported to possess intrinsically ultralow κ lat because of the local vibration of Cs and I atoms and dynamic octahedral rotation 263 . Figure 20(A) shows the calculated phonon dispersion of Cs 2 PbI 2 Cl 2 at its equilibrium structure.…”

“…(C) I atom vibration involved in the phonon mode at Γ the point (ω = 25 cm −1 ). (A–C) Reproduced with permission from Reference 263. Copyright 2020, American Chemical Society.…”

“…Insert refers to the relationship between measured direction and growth direction. Data in (D,E) were adopted from Reference 263…”

Microscopic mechanism of unusual lattice thermal transport in TlInTe2

“…Crystalline semiconductors with ultralow lattice thermal conductivity (κ l ) are important for effective utilization and management of thermal energy in high-performance thermoelectrics [1][2][3][4] , photovoltaics [5][6][7][8] , thermal barrier coatings 9 , and thermal data storage devices 10 . Although it is quite natural for compounds with complex crystal structures having large unit cells 11,12 and heavy atoms to exhibit low-κ l , relatively simple crystalline materials having small unit cells and even with light atoms 13 could also possess ultralow-κ l due to the presence of rattler atoms 14 , strong lattice anharmonicity [15][16][17][18][19] , or bonding heterogeneity [20][21][22][23] .…”

Two-dimensional halide perovskite single crystals: principles and promises

“…Recently, Acharyya et al studied RP 2D Cs 2 PbI 2 Cl 2 SCs by cutting them in the direction parallel and perpendicular to the Bridgman growth direction (Fig. 6a) [73]. First, stoichiometric amount of PbI 2 and CsCl was melted in the sealed quartz tube.…”