Phasing crystal structures from powder data: about the use of the Harker sections

Various authors have tried to exploit Harker sections to estimate the moduli and the phases of the one-phase structure seminvariants (s.s.) of the first rank from single crystal data. The estimates are sufficiently accurate if false Harker peaks are absent; therefore, if some heavy atoms are present in the structure, and this last is not too complex, the results are quite satisfactory. Such a technique has never been applied, until now, to powder data. In this case there is the basic advantage that the average crystal structure size is relatively small; on the contrary the unavoidable distortion of the Patterson map, due to the fact that this is calculated by using distorted structure factor magnitudes, is often severe.