Standard and Special Strategies in Structure Determination from Powder Data. Armel Le Bail, Université du Maine, Laboratoire des Fluorures, CNRS UPRES-A 6010, 72017 Le Mans, France.

A special strategy is defined here as an alternative when the usual way fails. In this sense, using one of the 30 or so synchrotron or neutron facilities is a special strategy : something to try if conventional X-rays did not succeeded or if a special accuracy was needed. Remember that more than 15000 conventional X-ray powder diffractometers, 2000 Guinier and 20000 Debye-Scherrer cameras may be found all around the world. The standard powder data strategy is very similar to a small molecule structure determination from single crystal data : one has successively to determine the cell, the space group, to measure the structure factors, to apply Patterson or direct methods, to complete the structure by Fourier difference syntheses and to refine it. This requires the best of crystallography softwares and a quite meticulous work. Lot of time can be wasted by errors in `automatic' cell and space group false propositions. There are two obvious reasons for failing in SDPD (Structure Determination from Powder Data), the first is that a powder is not always made from a pure compound (one sufficient single crystal picked in a mixture has no equivalent here) ; too low quality data is a second reason at one step or another, regarding the problem size. Hints have been gathered in a tutorial recently added to the SDPD-D [1] allowing a beginner to follow the standard strategy as applied to various cases. Applicability limits are related to diffractometer resolution, problem size and sample quality. Their knowledge leads now to admit that a SDPD is a routine task because failure can be easily predicted, sometimes by just looking at the pattern. Special strategies may require more sophisticated approaches than the above 'brute force' as the estimation of a partition of the strictly overlapping reflections other than equal, Bayesian analysis, rotation of known fragments inside the cell, model building, Monte Carlo moves up to obtain optimum atom position. Works using these approaches are currently few among the SDPD-D 300 cases. Availability in the public domain of softwares allowing these special data treatments may increase applications but they will logically represent a last chance when the standard easier strategy fails. A universal and cheap crystal data bank gathering all characterized compounds and allowing to calculate powder patterns is desirable for avoiding structure re-determination and useless efforts.

[1] Structure Determination from Powder Diffraction - Database, A. Le Bail, available at URL http://fluo.univ-lemans.fr:8001/iniref.html (1994-97).