Accurate determination of atomic displacement parameters for thermoelectric materials from synchrotron powder diffraction data with sophisticated background treatment

Abstract

We report on recent advances in the structural study of thermoelectric materials using synchrotron radiation (SR) powder X-ray diffraction, including a method for modeling background scattering. We show that analysis including first-order thermal diffuse scattering (TDS) improves the accuracy of atomic displacement parameters (ADPs) determined by Rietveld analysis. We have determined accurate anisotropic ADPs of Ag sites in the low-temperature phase of argyrodite Ag₈SnSe₆ using SR powder X-ray diffraction performed at SPring-8. Three of the five Ag sites, Ag1, Ag3, and Ag5, showed large and anisotropic ADPs. Our study confirmed the rattling in the low-temperature phase of Ag₈SnSe₆. The ADPs of Ag which showed large and anisotropic ADPs increased with a decrease in the area of the Se₃ triangle for all temperatures. The “retreat from stress” proposed for non-caged rattling was suggested as the mechanism of rattling of Ag₈SnSe₆.