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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.[3] 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.[6] However, more detailed studies based on single-crystal diffraction data almost always revealed cation disorder.[7] In the case of small scattering contrast, resonant X-ray diffraction corroborated the disorder.[8] 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.
[1] Harker, D. Z. Kristallogr. 1934, 89, 175-181.
[2] Pauling, L. Am. Mineral. 1975, 60, 994-997.
[3] Cook, N. J.; Ciobanu, C. L.; Stanley, C. J.; Paar, W. H.; Sundbald, K. Can. Mineral. 2007, 45, 417-435.
[4] Grauer, D. C.; Hor, Y. S.; Williams, A. J.; Cava, R. J. Mater. Res. Bull. 2009, 44, 1926–1929.
[5] Silkin, I. V.; Menshchikova, T. V.; Otrokov, M. M.; Eremeev, S. V.; Koroteev, Yu. M.; Vergniory, M. G.; Kuznetsov, V. M.; Chulkov, E. V. JETP Letters. 2012, 96, 322-325.
[6] Shu, H. W.; Jauilmes, S.; Flahaut, J. Solid State Chem., 1988, 74, 277-286..
[7] Karpinsky, O. G.; Shelimova, L. E.; Kretova, M. A.; Fleurial. J. P. J. Alloys Compd. 1998, 268, 112-117.
[8] Oeckler, O.; Schneider, M. M.; Fahrnbauer, F.; Vaughan, G. Solid State Sci. 2011, 13, 1157-1161.
