Here, we report a first-principles study of lattice vibrations and thermal transport in Cs2SnI6, the archetypal compound in the family of vacancy-ordered double perovskites. We show that twofold rattlers of...

Searching thermoelectric materials with high performance and low cost is now receiving special attention and great challenges in the field of material design. In this work, we perform first-principles lattice dynamics combined with temperature-induced anharmonic phonon renormalization and connected to the Boltzmann transport equation to predict thermoelectric performance in the cubic inorganic iodide perovskites CsBI3 (B = Pb, Sn, and Ge) at a high temperature of 700 K. Under stabilization of the cubic phase that exhibits strong anharmonic phonon modes at 0 K, our calculations show that at T = 700 K, these perovskites have ultralow lattice thermal conductivities below 0.6 W m–1 K–1 and high thermopower factors over 1.5 mW m–1 K–2, being comparable or superior to those of GeTe. Moreover, we find that cubic CsGeI3 and CsSnI3 have higher thermoelectric figure of merit ZT over 0.95 upon n-type doping, being attributed to the strong lattice anharmonicity and flat-dispersive electronic bands with high degeneracy.