Physical Insights on the Thermoelectric Performance of Cs 2 SnBr 6 with Ultralow Lattice Thermal Conductivity.

This study has investigated the microscopic mechanisms of ultralow lattice thermal conductivity by the first-principles density functional theory. By solving the phonon Boltzmann equation iteratively, we find that the thermal conductivity of the lattice is abnormally low and that glass like heat transfer behavior occurs. Therefore, in addition to the contribution about the particle-like propagation to heat transport, the off-diagonal elements of the heat-flux operator through wave-like interbranch tunneling of phonon modes are also considered. The results provided new insights into the minimum thermal conductivity (κ L ) for Cs 2 SnBr 6 (0.17 W m -1 K -1 at 450 K). It was also found that polar optical phonon scattering severely affects carrier lifetime. In addition, an impressive thermoelectric figure of merit of 0.55 at 450 K for Cs 2 SnBr 6 was obtained in the case of doping p -type carriers. The study helps us understand the ultralow κ L in complex crystals with strong anharmonicity and find that Cs 2 SnBr 6 is a new and promising thermoelectric material.