Single crystal neutron diffraction analysis of the cation distribution in the high-temperature phases α-Cu_2−xS, α-Cu_2−xSe, and α-Ag₂Se

Abstract

The distribution and thermal vibration of the mobile cations have been studied for α-Cu_2−xS (200°C), α-Cu_2−xSe (160, 210, and 360°C), and α-Ag₂Se (135, 150, 200, and 300°C). Both α-Cu_2−xS and α-Cu_2−xSe, space group Fm3m, have an fcc anion array within which anharmonically-vibrating Cu atoms are displaced from the centers of the tetrahedra towards the vacant octahedral interstices. α-Ag₂Se, space group Im3m, has Se in bcc packing and displays a cation distribution which is delocalized in channels along 〈100〉 similar to β-Ag₂S. Local maxima in the Ag density occur in pairs about the center of the tetrahedral interstice.