Simulation of layer-faulting in Nb2Co7intermetallic compound using DIFFaX+

Ambiguities in the structure determination of antimony tellurides arising from almost homometric structure models and stacking disorder

Ambiguities in the interpretation of both single-crystal and powder diffraction data can lead to wrong conclusions concerning the structure analysis of layered chalcogenides with interesting physical properties and potential applications. This is illustrated for binary and Pb-doped phases of the homologous series (Sb2)k(Sb2Te3)m. Almost homometric structure models for 39R-Sb10Te3[R\bar 3m,a = 4.2874 (6),c = 64.300 (16) Å,R1 = 0.0298] have been derived from initial structure solutions and crystal chemical considerations. The variation of the electron density on certain positions may further reduce the differences between the calculated diffraction patterns of non-congruent structure models as exemplified by the new compound 33R-[Sb0.978(3)Pb0.022(3)]8Te3[R\bar 3m,a = 4.2890 (10),c = 75.51 (2) Å,R1 = 0.0615]. Both compounds are long-range ordered, and in either case both `almost homometric' models can be refined equally well on experimental data sets. The models can only be distinguished by chemical analysis, as reasonable atom assignments lead to different compositions for each model. Interestingly, all structure solution attempts led to the wrong models in both cases. In addition, it is shown that stacking disorder of characteristic layers may lead to powder diffraction patterns that can be misinterpreted in terms of three-dimensional randomly disordered almost isotropic structures with a simple α-Hg-type basic structure.