A systematic procedure for the decomposition of a powder diffraction pattern

摘要

Ab initio crystal structure solution from powder diffraction data can be attained by means of the two-stage method: first the integrated intensities of individual reflections are extracted from the experimental profile; then the extracted intensities are processed by direct methods to recover their phase. The efficiency of the direct-methods process is strongly affected by the accuracy of the extracted intensities and this is a serious drawback for the solution of organic compounds, where the presence of light atoms leads to a rapid decrease of the scattering at high angles. In order to overcome this problem, a procedure is proposed which generates a set of different intensity partitions for groups of reflections in strong overlap: the resulting decomposition trials are supplied to direct methods. The procedure is able to reduce the phase error at the end of the phasing process by up to 50degrees and enables the solution of organic structures with data resolution not worse than 1.5 Angstrom in a reasonable computing time. The procedure has been implemented in the EXPO program, which makes use of the Le Bail extraction algorithm. [References: 35]