Twofold rattling mode-induced ultralow thermal conductivity in vacancy-ordered double perovskite Cs 2 SnI 6 .

We report a first-principles study of lattice vibrations and thermal transport in Cs 2 SnI 6 , the vacancy-ordered double perovskite. Twofold rattlers of Cs atoms and SnI6 clusters in Cs 2 SnI 6 , being different from CsSnI 3 with only Cs atom rattlers, largely scatter heat-carrying acoustic phonons strongly coupled with low-lying optical phonons and lower phonon group velocity. Using renormalized phonon dispersions at finite temperatures, we reveal that anharmonicity and twofold rattling modes induce an ultralow thermal conductivity at room temperature.