Rattling dynamics under a planar coordination in tetrahedrites

Suppressing lattice thermal conductivity (k_L) as low as possible is essential to achieve high thermoelectric performance. One of effective methods is to use the rattling which is large anharmonic vibration of atoms. Typically, rattling atoms locate in oversized atomic cages of caged compounds. Recently, we found several new thermoelectric materials that contain rattling atoms without oversized cages. in LaOBiSSe and tetrahedrites, Bi and Cu atoms having a planar coordination vibrate largely toward out of plane [1-3]. We investigated crystal structures and phonon dynamics of tetrahedrites to clarify the driving force for the occurrence of rattling in a planar coordination. We found that the amplitude of Cu rattling increases with decreasing S₃-triangle area. The rattling energy observed by inelastic neutron scattering decreases with decreasing the S₃-triangle area and finally damped demonstrating an enhancement of anharmonicity. The results suggest that chemical pressure is essential for the appearance of planar rattling in contrast to caged compounds where free space is essential.
[1] Y. Mizuguchi et al., J. Appl. Phys. 119, 155103 (2016). [2] C. H. Lee et al., Appl. Phys. Lett. 112, 023903 (2018). [3] K. Suekuni, C. H. Lee et al., Adv. Mater. 30, 1706230 (2018).