Twofold rattling mode-induced ultralow thermal conductivity in vacancy-ordered double perovskite Cs2SnI6

Using self-consistent phonon calculations, we reveal that strong anharmonicity and twofold rattling modes of Cs 2 SnI 6 induce an ultralow thermal conductivity of 0.11 W m −1 K −1 at 300 K, rarely expected of simple and single-crystalline solids.