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Crystal structure of a synthetic tin-selenium representative of the cylindrite structure type

E. Makovicky^1,*, V. Petíek², M. Duek² and D. Topa³

¹ Department of Geography and Geology, University of Copenhagen, Østervoldgade 10, DK1350 Copenhagen, Denmark
² Institute of Physics, Czech Academy of Sciences, Na Slovance 2, CZ-18040 Prague 8, Czech Republic
³ Department of Material Research and Physics, University of Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria

Correspondence: ^* E-mail: emilm{at}geol.ku.dk

The synthetic tin-selenium member of the cylindrite structural family, with the empirical formula Sn_31.52Sb_6.23Fe_3.12S_59.12 based on electron-microprobe data, has a triclinic crystal structure composed of two alternating layer types, both with a pronounced one-dimensional modulation, and with a non-commensurate layer match in two dimensions. The pseudotetragonal (Q) layer is a MeSe layer two-atomic planes thick with lattice parameters a = 5.969(2) Å, b = 6.004(1) Å, and the layer-stacking vector c = 12.238(1) Å, = 87.98(4)°, β = 83.14(3)°, and = 90.01(4)°. The pseudohexagonal (H) layer is a single-octahedral MeSe₂ layer with a = 3.831(1) Å, b = 6.580(3) Å, c = 12.151(5) Å, = 87.79(4)°, β = 90.59(3)°, and = 89.99(3)°; the a and b vectors of the two subsystems are parallel, the c vectors diverge. The transversal wave-like modulation has the wave-normal parallel to b, so that the modulation vector q is 0.0001(3) a* + 0.1921(4) b* – 0.0119(3) c* in terms of the pseudohexagonal subsystem. Superspace structure refinement in the superspace group X1 where X stands for non-primitive centering vectors (,,0,0,0), (0,0,0,0,), (,,0,0,) in five-dimensional superspace, and based on 2128 observed reflections, resulted in R₁ = 0.038 for all reflections. Composition of the H layer has been modeled as Sn₂₄₀⁴⁺Fe₅₄²⁺Se₅₈₈, that of the Q layer as Sn₃₀₆²⁺ Sb₁₀₈³⁺Se₄₁₄. The cation-anion distances in the Q layer vary between 2.63 and 3.30 Å, indicating that the cations present are primarily Sn²⁺ (and Sb³⁺), whereas distances in the H layer lie between 2.665 and 2.721 Å and correspond to Sn⁴⁺ with admixture of Fe²⁺. The shortest cation-anion distance across the interlayer space is 3.24 Å. Relations between layer match and the modulation vector, divergence of layer stackings of the two components, and reasons for the modulation and for the pronounced disorder of the Q component, as well as the differences and similarities with levyclaudite, franckeite, and synthetic layer-misfit compounds are discussed in detail. In its structural principles, although not in numerical values, the Sn-Se cylindrite corresponds fully to the natural Pb-Sn-S cylindrite previously described.

Key Words: Cylindrite • tin-antimony-iron selenide • non-commensurate layer structure • layer-misfit structure • interlayer match

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