Rattling of Ag Atoms Found in the Low-Temperature Phase of Thermoelectric Argyrodite Ag₈SnSe₆

A rattling of silver (Ag) atoms is identified in the low-temperature phase of Argyrodite compound Ag8SnSe6 through a systematic multitemperature synchrotron radiation powder diffraction study. Both Bragg and diffuse scatterings from the sample can be well explained by the presence of the Ag rattling atoms. The Rietveld refinements of the low-temperature phase were significantly improved by introducing anisotropic thermal motions of Ag1 and Ag3 sites. The diffuse scattering observed in the low-temperature phase can be well explained by a combination of the first-order thermal diffuse and liquid-like scatterings, which arise due to the rattling of Ag1 and Ag3 sites. The rattling of Ag1 and Ag3 sites is restricted in the perpendicular direction of the Se triangles. This behavior is very similar to that of rattling atoms in the tetrahedrite and tennantite. The present findings are consistent with the low thermal conductivity observed in the low-temperature phase and the existence of low-energy excitation due to neutron scattering.