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  • Phase Identification.

X-ray diffraction is one of the most powerful tools for identifying unknown crystalline phases. By comparing the positions and intensities of the diffraction peaks against a library of known crystalline materials, the target material can be identi fied. In addition, multiple phases in a sample can be identified and quantified. Even if one of the phases is amorphous, x-ray diffraction can determine the relative amount of each phase.

Several hundred thousand materials have been catalogued in various diffraction databases allowing us to identify both inorganic and organic substances. The International Centre for Diffraction Data (ICDD) produces The Powder Diffraction File (PDF) which contains, in its 2013 release, 778,883 unique material data sets. Each data set contains diffraction, crystallographic and bibliographic data, as well as experimental, instrument and sampling conditions.

To identify a particular phase both peak positions and relative intensities must fit. In general this requirement should hold for at least three peaks. Below is an example of how this process works. This example is a simple purity check on hydroxyapatite and similar logic applies to completely unknown samples.

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  • Phase Quantification.

The materials have their properties defined by the chemical composition and the microstructure presented on them. The quantification of crystalline phases is a key step in determining the structure, properties and applications of a given material. Therefore, the study of the amount of crystalline phases present in a material represents an important parameter to control the microstructure and the correlation of the properties associated with the developed stage in the process. Quantitative Phase Analysis using XRPD is a powerful method for determining the quantities of crystalline and amorphous components in multiphase mixtures.

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  • Crystallinity Index.

Many materials, especially polymers, tend to form both crystalline and non-crystalline forms. Each structure produces a unique diffraction pattern that enables us to determine the relative amounts of each phase. Amorphous phases produce one or more broad humps in the diffraction pattern, while crystalline materials produce a series of sharper peaks superimposed on the amorphous humps.

In order to perform this analysis,  one must know a great deal about all of the crystalline phases. If this information is not known, then the percent crystallinity will be estimated based on the relative areas of the amorphous and crystalline regions.

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Physics Division, National Research Center (NRC), Egypt.
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