DIRECT PHASING FROM POWDER DATA: THE EXTRA OPTIMIZED PROCEDURE C. Giacovazzo⁽⁺⁾, A. Altomare⁽⁺⁾, G. Cascarano⁽⁺⁾, A. Guagliardi⁽⁺⁾, 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.

Present difficulties in solving crystal structures from powder data are not to be found in the lack of efficiency of direct methods, but in the inaccuracy of the process which extracts structure factor amplitudes from a powder pattern. EXTRA [A. Altomare, M. C. Burla, G. Cascarano, C. Giacovazzo, A. Guagliardi, A. G. G. Moliterni & G. Polidori (1995), J. Appl. Cryst. 28,842-846] has been an efficient answer to this problem: friendly to use, the program is able to automatically decompose, via the Le Bail algorithm, quite complicated experimental patterns.

EXTRA is now incorporating new important features. It is able to take advantage from: a) the Patterson map; b) the pseudo-translational symmetry; c) the preferred orientation; d) the statistical estimates of the diffraction amplitudes via direct methods; e) a molecular fragment.

All the above sources of information are able to dramatically improve the accuracy of the pattern decomposition process so enlarging the size of crystal structures solvable by powder data. We are merging EXTRA with SIRPOW.92 [A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi. M.C. Burla, G. Polidori, & M. Camalli, (1994), J. Appl. Cryst. 27, 435-436], our direct methods program for powder data, to provide a unique program, transparent to the user, which should be able to solve a structure directly the diffraction pattern.