Crystallographic techniques structure factors

Crystallographers share the fruits of their work in the form of lists of atomic coordinates, which can be used to display and study the molecule with molecular graphics programs (Chapter 11). Less commonly, because fewer people have the resources to use them, crystallographers share the final structure factors, from which electron-density maps can be computed. The audience for structure factors includes other crystallographers developing new techniques of data handling, refinement, or map interpretation. 

The techniques of single-crystal diffractometry have been discussed by Arndt and WUlis (51). We should note that extinction is a very serious problem in the determination of accurate magnetic intensity data from single crystals. Although extinction must always be accounted for in conventional crystallographic studies, it is particularly important to make proper correction in polarized neutron experiments where the ratio of magnetic to nuclear structure factors is determined. 

As briefly mentioned in the previous chapter, the determination of a crystal structure may be considered complete only when multiple pattern variables and crystallographic parameters of a model have been fully refined against the observed powder diffraction data. Obviously, the refined model should remain reasonable from both physical and chemical standpoints. The refinement technique, most commonly employed today, is based on the idea suggested in the middle 1960 s by Rietveld. The essence of Rietveld s approach is that experimental powder diffraction data are utilized without extraction of the individual integrated intensities or the individual structure factors, and all structural and instrumental parameters are refined by fitting a calculated profile to the observed data. 

Refinement of the crystal structure is, therefore, a powerful chemical analysis technique. Unlike conventional chemical analysis, which only yields the bulk composition of the sample, powder diffraction analysis facilitates accurate determination of the occupancies of different crystallographic sites by various chemical elements, or in other words, establishes precise chemical composition of the crystal at the atomic resolution. It should be noted that the results may be considered reliable only when the difference in the scattering ability of atoms in question is significant, in addition to a very high quality of experimental data. This is indeed the case here because scattering factors of Sn and Ni are related as-1.8 1. 

---