Intrinsically Low Thermal Conductivity in the n-Type Vacancy-Ordered Double Perovskite Cs 2 SnI 6 : Octahedral Rotation and Anharmonic Rattling

Fundamental understanding of the relationship between chemical bonding, lattice dynamics, and thermal transport is not only crucial for thermoelectrics but also essential in photovoltaics and optoelectronics. This leads to a widespread search for low thermally conductive crystalline metal halide perovskites with improved electrical transport and stability. Pb-free all-inorganic Sn-based halide perovskites are particularly compelling because of their degenerate hole doping capability, which generally results in p-type conduction. Herein, we demonstrate an n-type thermoelectric conduction in concurrence with an ultralow lattice thermal conductivity (κ lat ∼0.29–0.22 W/m·K) in an air-stable vacancy-ordered double perovskite Cs 2 SnI 6 . Phonon dispersion calculated by density functional theory indicates the presence of low-frequency localized optical modes at 8 and 32 cm –1 due to the dynamical rotation of SnI 6 octahedra and anharmonic rattling of Cs-atoms, respectively, which are experimentally verified by temperature-dependent Raman spectroscopy and low-temperature heat capacity measurement. Cs 2 SnI 6 exhibits a soft elastic lattice with chemical bonding hierarchy that causes low bulk and shear moduli, which in turn results in a low measured sound velocity of ∼1158 m/s. Low-energy anharmonic optical modes strongly couple with heat-carrying acoustic phonons and, consequently, limit phonon group velocity and phonon lifetime to an ultrashort value, leading to an intrinsically ultralow κ lat in n-type Cs 2 SnI 6 .