The crystal structure of the mineral tetradymite Bi2S2Te has inspired discussions about its details for a long time.[1,2] Variations of this structure led to a large family of chalcogenides composed of slabs with alternating cation and anion layers stacked in rocksalt-type fashion, which show characteristic van der Waals gaps between anion layers. These compounds are of interest not only for their exact crystal structures but also for their thermoelectric properties and their high likelihood of being topologically non-trivial systems insulators.[4,5]
The rhombohedral compounds Tt1Pn2Te4 (Tt = Ge, Sn, Pb; Pn = As, Sb, Bi) feature septuple layers, in which the multiplicity of Wyckoff positions would allow complete cation ordering, which has been postulated in some cases. However, more detailed studies based on single-crystal diffraction data almost always revealed cation disorder. In the case of small scattering contrast, resonant X-ray diffraction corroborated the disorder. Here we present a systematic investigation of these compounds with high-quality single crystal diffraction data collected with synchrotron radiation in order to establish exact site occupancy factors. As diffraction data cannot exclude short-range ordering, Z-contrast imaging by STEM-HAADF was used as a local probe, complemented by EDX spectroscopy with atomic resolution. All of these data confirm the cation disorder and could not even demonstrate short-range ordering.
The disorder observed might be explained as an interplay between charge balance at the vdW-gap and octahedron size mismatch in the layer.
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