Single crystal software powder diffraction

Compared to powder diffraction software, single crystal refinement programs are currently more advanced for routine structure refinement (Table 17.25). Compared to existing Rietveld software, it can be less time consuming to use a... 

While overshadowed by dedicated powder diffraction based structure solution software, single crystal structure solution programs can still be used where a list of HKLs and intensities are available and can be usefully applied (Table 17.23). This allows not only the use of Direct Methods, but also Patterson and Patterson-based fragment searching. Armel Le Bail s Overlap program (listed in Table 17.16) can remove reflections that are excessively overlapped, sometimes aiding single crystal software based structure solution. 

A first port of call for checking out available crystallographic software. Links to and mirrors of single crystal and powder crystallographic software. Includes tutorials for some of the software and lists of available software Free CD ROM containing various Internet available single crystal and powder diffraction software and resources. Mainly intended for academics and students in the developing world... 

In order to obtain detailed structure, a knowledge of diffraction intensities is essential, the intensities being related to the structure factor. Computer-controlled single-crystal X-ray diffractometers with structure (software) packages have made structure elucidation a routine matter. The availability of synchrotron X-radiation of continuously variable wavelength has made X-ray diffraction a still more powerful structural tool for the study of solids. A technique of great utility to solid state chemists is the Rietveld treatment of powder X-ray diffraction profiles (Rietveld, 1969 Manohar, 1983). Automated structure packages for the determination of unknown structures by this method are now commercially available (see section 2.2.3). In Fig. 2.1, we show a typical set of profile data. 

Qualitative and quantitative analytical applications of X-ray diffraction both require reference diffraction patterns to identify and quantify the different polymorphic modifications. Experimental powder patterns may be suspect for their use as standards as a result of experimentally induced errors or aberrations or the lack of polymorphic purity in the sample itself (which may even result from the sample preparation). The availability of full crystal structure determinations for any or all of the polymorphic modifications can considerably facilitate generation of standard powder patterns. A variety of public domain software is now available for calculating powder diffraction patterns from single crystal data (ICDD 2001, lUCr 2001)." ... 

The most comprehensive collections of various crystallographic software products are found at the International Union of Crystallography Web site at http //www.iucr.org and at the Collaborative Computational Project No. 14 for Single Crystal and Powder Diffraction Web site at http //www.ccpl4.ac.uk. 

While computer-based structure validation is integrated in single crystal software, it is somewhat lagging in the powder diffraction community. Ton Spek s... 

Although, the powder method was developed as early as 1916 by Debye and Scherrer, for more than 50 years its use was almost exclusively limited to qualitative and semi-quantitative phase analysis and macroscopic stress measurements. The main reason for this can be found in what is known as the principal problem of powder diffraction accidental and systematic peak overlap caused by a projection of three-dimensional reciprocal space on to the one-dimensional 26 axis, leading to a strongly reduced information content compared to a single crystal data set. However, despite the loss of angular information, often sufficient information resides in the ID dataset to reconstruct the 3D structure. Indeed, quantitative analysis of the pattern using modern computers and software yields the wealth of additional information about the sample structure that is illustrated in Figure 1. Modern... 

Improvements in hardware have been accompanied by enhancements to software that facilitate the refinement of even more complex strnctures. Techniques that have been developed for powder diffraction and protein crystallography can now be applied routinely to single-crystal analysis and vice versa. For single-crystal studies, this has led to more reliable refinements of weak, low-resolution datasets that are unavoidable when, as is often the case in supramolecular chemistry, only microscopic crystals are available. 

All samples were single phase without detectable impurities. The crystal structure was refined by the Rietveld method (Rietveld, 1959) from X-ray powder diffraction data using GSAS software package (Larson Von Dreele, 1994). 

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