Effect of Planar Rattling on Suppression of Thermal Conductivity in Thermoelectric Materials

We studied the thermoelectric properties, crystal structure, and phonon dynamics of LaOBiS2-xSex. We found that LaOBiS2-xSex has an extremely low lattice thermal conductivity of 1 W/mK around room temperature, which is lower than that of vitreous silica. This low lattice thermal conductivity indicates that LaOBiS2-xSex is a promising thermoelectric material with a dimensionless figure of merit of 0.35 at T = 650 K. Crystal structure refinement revealed that Bi atoms vibrate considerably along the c-axis. The energy of Bi vibration modes decreases linearly with decreasing lattice thermal conductivity, suggesting that the Bi vibration modes are responsible for the low lattice thermal conductivity, similar to rattling in oversized cage compounds. The present Bi rattling modes are activated by in-plane chemical pressure, in contrast to those in oversized cage compounds where a larger free space enhances the rattling modes further. We defined this new type of rattling as planar rattling, which can provide a new strategy for developing high-performance thermoelectric materials.