Phase identification isostructural

Every crystalline phase in a sample has a unique powder diffraction pattern determined from the unit cell dimensions and the atomic arrangement within the unit cell. It can be considered a fingerprint of the material. Thus, powder diffraction can be used for phase identification by comparing measured data with diffraction diagrams from known phases. The most efficient computer searchable crystallographic database is the PDF-4 from the International Centre for Diffraction Data (ICDD) [3]. It is used by very efficient computer-based search-processes. In 2007 the PDF-4-i- database contains information about Bragg-positions and X-ray intensities for more than 450000 compounds, out of which there are about 107 500 data sets with atomic coordinates. New entries are added every year. The positions of the peaks in the measured pattern have to be determined. This can be done manually, but effective, fast and reliable automatic peak search methods have been developed. The method can obviously be successful only if the phases in the sample are included in the database. However, the database can also help to determine unknown phases if X-ray data exist for another isostructural compound albeit with a different composition. 

The identification, structural and thermal characterization of new polymorphs is an important topic in solid-state chemistry and requires a battery of techniques that includes X-ray diffraction and spectroscopic methods, in addition to thermal analysis methods and dissolution techniques to determine solubility trends. Such studies are described by Caira in Chapter 16, as well as more recent theoretical techniques aimed at the prediction of the crystal structures of new polymorphs. Crystal polymorphism is particularly important in pharmaceutical products, so there is an emphasis on this area. Systems displaying solvatomorphism (the ability of a substance to exist in two or more crystalline phases arising from differences in their solvation states) molecular inclusion and isostructurality (the inverse of polymorphism) are also given due attention in this chapter. 

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