The positivity of the Patterson function is used as prior information for the decomposition of a powder diffraction pattern. An automatic procedure is described which, integrated with the Le Bail method, is able to provide estimates of IFI values that are often remarkably better than those obtained by application of standard techniques. The procedure, implemented in a computer program, has been applied to a set of test structures (experimental data); the results show that, particularly for structures with some heavy atoms, the new estimates of the structure-factor moduli are far better (on average) than those obtained via the default application of EXTRA [Altomare et al. (1995). J. Appl. Cryst. 28,[842][843][844][845][846].

The principal limitation of the diffraction methods for crystal structure analysis from powder data is originated by the collapse of the three-dimensional reciprocal space into the one dimension of the powder diffraction pattern. The degradation of the information can make difficult even the solution of small crystal structures and can generate inefficiencies in the least-squares methods devoted to crystal structure refinement. In this paper, the current two-stage procedures, the first stage dedicated to powder-pattern decomposition and the second to direct phasing of powder data, are analysed. It is shown that in the first stage such procedures disregard a large amount of information that can become available during the process of crystal structure solution and analysis. The use of such information is essential for making directmethods procedures more robust and for improving the accuracy of the least-squares techniques. The performances of EXTRA [Altomare, Burla, Cascarano, Giacovazzo, Guagliardi, Molitemi & Polidori (1995). J.Appl. Cryst. 28, 842--846], a program for full-pattern decomposition based on the Le Bail algorithm, and of SIRPOW.92 [Altomare, Burla, Cascarano, Giacovazzo, Guagliardi, Polidori & Camalli (1994). J. Appl. Cryst. 27,[435][436], a direct-methods program optimized for powder data, are discussed in order to offer to the reader a logical pathway for the analysis of the traditional techniques and for the proposition of a new approach. It is shown that pattern-decomposition programs based on the Le Bail algorithm are able to exploit the prior information in a more effective way than Pawleymethod-based decomposition programs.

In direct procedures for crystal structure solution from powder data, information on the location and orientation of a molecular fragment may readily become available. Such information may be used retrospectively to improve the powder‐pattern decomposition, with favourable effects on the phasing process. A method is described by which accurate estimation of a large number of structure‐factor moduli is possible by exploiting the prior partial structural information.

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