Iterative SP-FFT phase refinement procedure for powder data. Only the differences with respect to the already described S-FFT algorithm are indicated (Fig. 3). Initial phase values (upper right corner) are combined with weighted experimental and extrapolated amplitudes to give the initial ρ values (upper right corner). The Fourier coefficients of the synthesis are obtained by combining the experimental (IH − 〈I〉) intensities with the ψ phases. The new structure factor estimates are alternatively obtained either by Fourier transforming δ·ρ (FT) or directly from the N top-ranked Fourier peaks (SFC) of δ·ρ. For overlapped reflections, values are updated every cycle [tH is the intensity redistribution coefficient calculated according to equation (33), which ensures the constancy of the global cluster intensity].