conference abstracts s350 ECM Nancy Acta Cryst. (2000). A56 (Supplement), s350 s10.m1.p5 Advances in automatic powder indexing: An enhanced 32-bit version of the Crysfire suite.
Recently released powder indexing programs are reviewed and placed in competition with the established programs (ITO, TREOR, DICVOL, etc.) through a series of problems selected among previously unindexed ICDD entries designated as “high quality”. Benchmarks are provided for testing indexing programs, based on the bethanechol chloride powder diffraction data. Applying these benchmarks leads to a classification (with respect to this specific example) of indexing programs as they face progressively more difficult situations. High data quality and the user experience to obtain it are concluded to remain the best way to indexing success, given that nearly all programs produce excellent results with excellent data. Lack of attention to data quality, even if followed by use of the most efficient programs, will usually lead to failure. It is demonstrated how not restricting oneself to a single indexing program can considerably increase the chances of success.
Unit-cell dimensions and optical data for uric acid and uric acid dihydrate show that there are structural resemblances, but that either can be readily distinguished by x-ray methods or by optical techniques. Density separation may be unsatisfactory because the dihydrate can lose water rather easily. Powder diffraction data are given.
The crystal and molecular structure of tetraiodoethylene (C214) at 4 K has been determined by neutron powder diffraction with Rietveld's profile-refinement method. Three independent data sets at different wavelengths were collected and used in the calculations. The measurements were sufficiently accurate to locate the C atoms in the molecules, and to show that the C=C bonds in both molecules are fully ordered and aligned approximately parallel to a*. The dimensions of the unit cell at 4 K and several interatomic distances are reported. The estimated standard deviations derived by this method are discussed and a normalization and weighting scheme for combining results from multiple power-data sets is suggested.1765
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