Unraveling the Crystal Structures of Ag₂Ch (Ch = S, Se, Te) via Simultaneous X-Ray Diffraction and Total Scattering

Despite great interest in the properties of the Ag₂Ch (Ch = S, Se, Te) compounds, their complex crystal structures, envisioned as a disordered liquid-like Ag substructure, are poorly understood. This limits our understanding of the structural origin of their attractive properties. From combined powder X-ray diffraction (PXRD) and X-ray total scattering (TS) in the temperature range of 300-800 K, the average and local structures for all three Ag₂Ch compounds have been probed simultaneously for both the room-temperature (RT) and first high-temperature (HT) polymorphs. From Rietveld refinement of the PXRD data, large and highly anisotropic atomic displacement parameters were identified for Ag in all RT polymorphs, which cannot be attributed solely to thermal displacement. Furthermore, the Rietveld models struggle with describing the highly diffuse distribution of Ag in the HT polymorphs, especially for Ag₂S and Ag₂Se, for which the maximum entropy method (MEM) is better suited. By comparing pair distribution functions (PDFs), it is found that the local structure cannot be described by the periodic Rietveld model alone at any temperature. New structural descriptions are proposed to account for these observations, in which Ag is displacively disordered in the RT polymorphs, and the lattice is locally distorted in the HT polymorphs to resemble the RT structure due to strong Ag-Ch interactions. Thus, the chalcogenide framework is far from being rigid, the Ag substructure is far from being liquid, and both are governed by specific local chemical interactions.