Two-dimensionally modulated structure of the rare-earth polysulfide GdS2−x (x = 0.18 ≃ 13/72)
Two-dimensionally modulated structure of the rare-earth polysulfide GdS2−x (x = 0.18 ≃ 13/72)
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稀土多硫化物 GdS2−x 的二维调制结构 (x = 0.18 ≃ 13/72)
DOI：
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发表时间：
2003
期刊：
影响因子：
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通讯作者：
H. Arnold
中科院分区：
文献类型：
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作者：
R. Tamazyan;S. van Smaalen;I. Vasilyeva;H. Arnold
GdS2−x的晶体结构由单晶x射线衍射确定为ZrSSi结构型的144倍上层结构。超空间群为P4/n(αβ 1/ 2)(00)(ss), α = 1/4, β = 1/3。在经典方法中，使用144倍超级单体的结构改进失败了，因为在实验分辨率内，大多数超晶格反射的强度为零。在超空间方法中，无超晶格反射被系统地归类为高阶卫星反射。因此，利用超空间方法对包含三个晶体独立原子的基本结构位置和调制函数振幅的结构模型进行了改进。衍射数据的拟合质量和精化参数的值与真正对称的假设无关（不相称或具有不同对称性的12 × 12 × 2， i中心超晶格）。结构合理性的论证表明，真正的结构是一个具有空间群I\bar{4}的超结构，对应于由（t1, t2）给出的超空间部分等于[(4n−1)/48,(4m−3)/48]或[(4n−3)/48,(4m−1)/48]（n和m是整数）。利用超空间技术（t图）和超级单体结构模型对结构进行分析，结果表明，在S2原子的方形层内，超空间结构对应于空位的有序和S_2^{2-}二聚体的取向有序。
The crystal structure of GdS2−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 P4/n(αβ½)(00)(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, I-centred superlattice with different symmetries). Arguments of structural plausibility then suggest that the true structure is a superstructure with space group I\bar{4}, corresponding to sections of superspace given by (t1, t2) equal to [(4n − 1)/48, (4m − 3)/48] or [(4n − 3)/48, (4m − 1)/48] (n and m are integers). Analysis of the structure, employing both superspace techniques (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.