超空间群理论对“游标卡尺结构”的应用

本文用dewolff等人发展的超空间群理论,对具有“游标卡尺结构”的Ba_1+xFe₂S₄和R_1+εFe₄B₄化合物的对称性进行了描述,两者的超对称性分别由P_1Iss^I4/mmm与P_1Iss^(P4₂/ncm)表示。通过在超空间中系统消光规律的讨论,解释了“游标卡尺结构”在公度的长周期超结构模型下那些和三维空间间群无关的消光规律,并和已知的衍射数据进行了比较,在进行“游标卡尺结构”的结构测定时,这些衍射选择定则有助于判断哪些位置可能存在可测的衍射强度。 关键词：

THE APPLICATION OF THE SUPERSPACE GROUP ON THE VERNIER STRUCTURES

The nonstoichiometric compound Ba_1+xFe₂S₄(0.062≤x≤0.143) and the rare earth-transition rnetal tetraborides R_1+εFe₄B₄(ε≈ 0.1, R = Ce, Pr, Nd, Sm, Gd, Tb), both composed of two interpenetrating substructures, are examples of the vernier structures. In present paper, the symmetry of the crystal structure in these two systems is characterized by the superspace group approach developed by de Wolff et al. The superspace group for Ba_1+xFe₂S₄ compound is P_1Iss^{I4/mmm and the one for all R_1+εFe₄B₄ compounds is P_1Iss(P4₂/ncm). The diffraction selection rules in the commensurate models for the vernier structures, those often occur and are not related to the systematic absences of the three-dimensional space groups, are explained according to the results of the systematic absences discussed in the superspace. Such diffraction selection rules are very useful to predict where the measurable intensities can be found in the case of crystal structure analysis for the vernier structures.}