A systematic procedure for the decomposition of a powder diffraction pattern

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 50° and enables the solution of organic structures with data resolution not worse than 1.5 Å in a reasonable computing time. The procedure has been implemented in the EXPO program, which makes use of the Le Bail extraction algorithm.