Origin of ultra-low thermal conductivity in Ruddlesden Popper 2D Perovskite Cs2PbI2Cl2: An Inelastic Neutron Scattering Study

Crystalline solid with low thermal conductivity is crucial for high-performance thermoelectrics. Recently, we have demonstrated an intrinsically ultralow lattice thermal conductivity (κL) in 2D Ruddlesden−Popper (RP) perovskites Cs2PbI2Cl2. We have observed intrinsically ultralow κL of ∼0.37−0.28 W/mK in the 295−523 K temperature range when measured along the crystallographic c-axis. First-principles density functional theory analysis of the phonon spectrum exhibits low energy optical mode at ∼12 cm−1 (1.5 meV) that originates from dynamic octahedral rotation around Pb caused by anharmonic vibration of Cl atoms induced by a 3s2 lone pair. Inelastic Neutron Scattering (INS) data as a function of temperature will enable us to extract these low energy optical modes, their phonon lifetimes, etc. and help us to investigate the fundamental cause for the intrinsically ultralow κL in Cs2PbI2Cl2.