SOLVING CRYSTAL STRUCTURES FROM POWDER DATA: EXTRA AND SIRPOW PACKAGES. M.C. Burla^(*), A. Altomare⁽⁺⁾,G. Cascarano⁽⁺⁾, C. Giacovazzo⁽⁺⁾, A. Guagliardi⁽⁺⁾, A.G.G. Moliterni⁽⁺⁾ & G. Polidori ^(*)

^(*)Dipartimento di Scienze della Terra, Universita, 06100 Perugia, Italy. ⁽⁺⁾Istituto di Ricerca per lo Sviluppo di Metodologie Cristallografiche - CNR c/o Dipartimento Geomineralogico - Campus Universitario Via E. Orabona, 4 - 70125 Bari - Italy.

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.

Current decomposition methods disregard a large amount of information which can become available during the process for crystal structure solution and analysis. The use of such an information is essential for making direct methods procedures more robust and for improving the accuracy of the least-squares techniques. EXTRA [Altomare A., Burla M.C., Cascarano G., Giacovazzo C., Guagliardi A., Moliterni A.G.G. & Polidori G., (1995), J. Appl. Cryst. 28, 842-846], a program for full pattern decomposition based on the Le Bail algorithm, is able to exploit the following types of prior information:

1) the positivity of the electron density through the Patterson function.

2) the pseudo-translational symmetry, which becomes available as soon as normalized structure factors are calculated.

3) the preferred orientation. Also this information can be obtained by a suitable analysis of the normalized structure factors.

4) the partial structure. This information becomes available as soon as a reliable structure fragment is provided in the first E-map.