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The Dupont Powder Challenge

[R. L. Harlow]

The structures of micro-crystalline compounds can sometimes be solved directly from their synchrotron powder diffraction patterns. However, the odds decrease as peaks broaden, as unit cell dimensions and the size of the asymmetric unit increase, and as the structural symmetry decreases. Techniques derived from single-crystal methods do not work well with powder data taken on samples with these "complications". For the powder technique to be generally useful as a routine method of crystal structure analysis, new techniques for solving structures need to be found. A variety of new techniques are being explored. In an effort to determine which of these techniques might be useful for DuPont's problems. the Corporate Center for Analytical Science (CCAS) has set aside a US$ 1000 prize to the first person/group who can solve the crystal structure of HAlF4 [Chem. of Materials, 7, 75 (1995)]. The preparation of this compoundinvolves moderate heating of aluminum fluoride salts which contain organic cations. The peaks in the X-ray powder diffraction patterns are broad. Attempts to anneal the compound result in decomposition to AlFl3. Neither of two high-resolution, synchrotron X-ray powder diffraction patterns on this material have produced a structure using traditional techniques available at DuPont. The challenge is now open to the crystallographic community. Two sets of data and additional details have been posted (http://www.pitt.edu/-geib/powder.html).

The correctness of any proposed model will be tested against a neutron diffraction pattern (HFBR - HRNPD), not available to the contestants. The first model which makes chemical sense (coordination numbers. bond values and bond-valence sum test) and matches the X-ray and neutron diffraction patterns [with R(Bragg) below 0.08] will be declared the winner and the authors of the model will be entitled to the US$ 1000. Proposed structural solutions should be sent to R. L. Harlow, CRD, E228/316d, The DuPont Co., Wilmington, DE, 19880-0228, USA.

Good Luck.

R. L. Harlow