Intrinsic ultralow lattice thermal conductivity in lead-free halide perovskites Cs3Bi2X9 (X = Br, I)

The rattling vibrations of the Cs atoms as well as the rotational and torsional vibration modes of the polyhedral framework induce the ultralow lattice thermal conductivities of Cs 3 Bi 2 X 9 (X = Br, I).