Direct Methods and Powder Data: State of the Art and Perspectives

Solving crystal structures by applying direct methods to single-crystal data is a relatively easy task for structures with up to 100 atoms in the asymmetric unit. Their successful application to powder data is still a challenge unless the size of the structure is moderate. The rate of success depends on several factors like the efficiency of the full-pattern-decomposition programs, the peak overlapping, the presence of preferred orientation in the powder specimen, the nature of the background, the type of radiation used for collecting experimental data etc. The main factors are analysed in order to provide a clear description of the specific problems that have to be forced when direct phasing from powder data is attempted.