Two-dimensionally modulated structure of the rare-earth polysulfide GdS 2-x (x = 0.18 ≃ 13/72)

Titelangaben

Tamazyan, Rafael ; van Smaalen, Sander ; Vasilyeva, Inga Grigorevna ; Arnold, Heinrich:
Two-dimensionally modulated structure of the rare-earth polysulfide GdS 2-x (x = 0.18 ≃ 13/72).
In: Acta Crystallographica Section B. Bd. 59 (2003) Heft 6 . - S. 709-719.
ISSN 2052-5206
DOI: https://doi.org/10.1107/S0108768103022213

Volltext

Link zum Volltext (externe URL):

Abstract

The crystal structure of GdS\sb 2-}\it x} is determined by single-crystal X-ray diffraction as a 144-fold superstructure of the ZrSSi structure type. The superstructure is described as a two-dimensional, commensurately modulated structure with the superspace group \it P4/\it n(αβ\scriptstyle1øver2)(00)(\it ss) and with α = 1/4 and β = 1/3. Structure refinements within the classical approach, employing the 144-fold supercell, fail because most of the superlattice reflections have zero intensities within the experimental resolution. Within the superspace approach the absent superlattice reflections are systematically classified as higher-order satellite reflections. Accordingly, the superspace approach has been used to refine the structure model comprising the basic structure positions and the amplitudes of the modulation functions of the three crystallographically independent atoms. The quality of fit to the diffraction data and the values of the refined parameters are independent of the assumption on the true symmetry (incommensurate or a 12×12×2, \it I-centred superlattice with different symmetries). Arguments of structural plausibility then suggest that the true structure is a superstructure with space group \it I\bar4, corresponding to sections of superspace given by (\it t\sb 1, \it t\sb 2) equal to (4{\it n} {$-$} 1)/48, (4{\it m} {$-$} 3)/48 or (4{\it n} {$-$} 3)/48, (4{\it m} {$-$} 1)/48 ({\it n} and {\it m} are integers). Analysis of the structure, employing both superspace techniques ({\it t} plots) and the supercell structure model all show that the superstructure corresponds to an ordering of vacancies and an orientational ordering of S_2^{2-} dimers within the square layers of the S2 atoms.