ACCURACY OF STRUCTURE FACTOR AMPLITUDES FROM A POWDER DIFFRACTION PATTERN: THE ROLE OF THE PRIOR INFORMATION. A. Guagliardi⁽⁺⁾, A. Altomare⁽⁺⁾, G. Cascarano⁽⁺⁾, C. Giacovazzo⁽⁺⁾, A.G.G. Moliterni⁽⁺⁾, M.C. Burla^(*) & G. Polidori^(*), ⁽⁺⁾Istituto di Ricerca per lo Sviluppo di Metodologie Cristallografiche - CNR c/o Dipartimento Geomineralogico - Campus Universitario Via E. Orabona, 4 - 70125 Bari - Italy,^(*) Dipartimento di Scienze della Terra, Universita', 06100 Perugia, Italy

The two-stage method is often used for analysis and crystal structure solution from powder diffraction data. In the first stage the integrated intensity of each Bragg reflection is obtained by a pattern decomposition process; the second stage uses these intensities to solve the structure by Direct or Patterson Methods.

The accuracy of structure factor amplitudes supplied by the first step is often not sufficient for the success of such an approach.

Besides the quality of the experimental pattern and the efficiency of the extraction procedures, the possibility of exploiting some prior information in the Le Bail algorithm [Le Bail, A., Duroy, H. & Fourquet, J.L. (1988). Math. Res. Bull. 23,447-452] greatly contributes to improve the accuracy of structure factor moduli. Different kinds of prior information will be discussed and used and the optimized algorithms for their management will be described in detail.