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Sven Lidin, F.E. Rohrer, H. Lind, L. Eriksson, A.-K. Larsson

On the question of commensurability – The Nowotny chimney-ladder structures revisited

A composite approach was used for the structure solution and refinement of two new Nowotny chimney-ladder phases, (Mo/Rh)₁₁Ge₁₈ and (Mo/Rh)₁₃Ge₂₂, found during investigations in the Mo/Rh/Ge system. The descriptions of the phases with conventional models (in I¯42d(No. 122) with a=5.885(2) Å, c=52.62(2) Å for (Mo/Rh)₁₁Ge₁₈ and a=5.922(1) Å, c=62.88(4) Å for (Mo/Rh)₁₃Ge₂₂) led to high numbers of unobserved reflections, high parameter/reflection ratios and an unstable refinement in the case of (Mo/Rh)₁₃Ge₂₂. Microscopy studies showed that the phases were in fact incommensurately modulated. Therefore, a composite model consisting of a rather stable diamond-like transition-metal substructure and a helical main-group-metal arrangement was developed. For the refinement of both phases, (Mo/Rh)₁₁Ge₁₈ as well as (Mo/Rh)₁₃Ge₂₂, the same model – same space group pair (W: P4/nnc:q¯1q1 P: I4₁/amd: 1¯1ss), same atom positions and modulation waves – was used. The refinements converged rapidly, showing that the presented structure model is very general, and may well be applicable to any Nowotny chimney-ladder phase.

Zeitschrift für Kristallographie, Oldenbourg Wissenschaftsverlag

Print ISSN: 0044-2968
Volume: 215, 11/2000
Pages: 650

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