低级晶系Patterson法多解的一般形式

应用Patterson法求解含有重原子的晶体结构时,时常由于重原子具有了高于整个结构的赝对称性而导致轻原子位置的多解(双解、四解乃至八解)。而轻原子位置的多解,在衍射空间中则表现为一定数量的衍射结构振幅分量符号的不确定问题。本文分析了低级晶系(三斜、单斜与正交晶系)所属的74个空间群,对于一个结构不对称单位中含有一个重原子的情况,获得了导致轻原子多解的重原子可能位置、赝对称性、轻原子多解形式、衍射结构振幅分量的分布规律以及应用直接法解决这类问题的具体途径。 关键词：

THE GENERAL TYPES OF MULTIPLE SOLUTION IN PATTERSON METHOD FOR LOW SYMMETRY SYSTEMS

In solving the crystal structures containing heavy atoms by patterson method, if the heavy atoms have a pseudo-symmetry which is higher than that of the whole structure, this will result in multi-solution positions of light atoms (two, four or eight possible positions). Expressed in reciprocal space, this problem is equivalent tc that some of the component signs of the diffraction structure amplitudes are undetermined. Seventy four space groups of low symmetry systems (trielinic, mono-clinic and orthorhombic systems) have been analysed. For an asymmetric unit involving only one heavy atom, with the possible positions of the heavy atom and the pseudo-symmetry given, the multi-solution types of light atoms and the distribution rule for components of diffraction struture amplitudes are obtained. In addition, a possible way for resolving these problems by direct method is proposed.