直接法中相位退化问题的处理

相位退化是直接法应用中的一大问题。所谓相位退化,系指非中心对称晶体结构因子的相位在推引或修正过程中收敛为中心对称类型的相位,属于P2₁、P4₁等空间群的晶体,常会出现这一情况。相位退化使相应的E图添加了一个对称中心,这使E图中“鬼峰”的数目大增,因而往往导致直接法的失败。本文提出一种两步处理方法:先将晶体当作中心对称求解以获得一个同时包含真正结构及其对映体的、中心对称的赝结构;然后在此基础上利用所谓分量关系式从赝结构解出真正的结构。此法经用已知结构的晶体检验,证明效果良好。

PROCEDURE FOR SOLVING THE PROBLEM OF PHASE DEGENERATION RESULTING FROM DIRECT METHODS

A serious problem in the application of direct methods is the problem of phase degeneration. That is the phases of structure factors of a non-centrosymmetric crystal converge to a set of centrosymmetric phases. This would occur frequently in case of space groups P2₁, P4₁ etc. Phase degeneration gives rise to an additional centre of symmetry and hence a considerable number of spurious peaks in the resulting E-map leading often to the failure of direct methods. In this paper, a two step procedure of solving this ambiguity is proposed. In the first step, the crystal is treated as a centrosymmetric one and a mised image including both enantiomorphs is obtained, while in the second step, the actual structure is then derived from the mixed image by making use of the so-called "component relations". This method has been verified with a known structure and proved to be effective.