Crystalline phases, fingerprinting

Powder diffraction patterns have three main features that can be measured t5 -spacings, peak intensities, and peak shapes. Because these patterns ate a characteristic fingerprint for each crystalline phase, a computer can quickly compare the measured pattern with a standard pattern from its database and recommend the best match. Whereas the measurement of t5 -spacings is quite straightforward, the determination of peak intensities can be influenced by sample preparation. Any preferred orientation, or presence of several larger crystals in the sample, makes the interpretation of the intensity data difficult. 

A basic, yet crucially important, application of powder XRD is in the identification ( fingerprinting ) of crystalline phases, based on the fact that different crystal structures give rise to distinct powder XRD patterns. Qualitative characterization of materials in this manner finds applications in many scientific disciplines (both academic and industrial), including quality control, polymorph screening, and the characterization of products from rapid throughput crystallization experiments [97, 98]. 

Each powder diffraction pattern is characterized by a unique distribution of both positions and intensities of Bragg peaks, where peak positions are defined by the unit cell dimensions and reflection intensities are established by the distribution of atoms in the unit cell of every crystalline phase present in the sample (see Table 2.7 in Chapter 2). Thus, every individual crystalline compound has its own fingerprint , which enables the utilization of powder diffraction data in phase identification. ... 

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 2D symmetry yields information of the projected crystal potential, giving identification of the crystalline phases present in a material or powder. The special characteristics of this method make it possible to use the CBED patterns as fingerprints for identification of phases [6]. 

Techniques for differentiating between amorphous and crystalline are (i) sharp melting point, (ii) sharp peaks in the solid state infrared fingerprint region, (iii) optical birefringence observed when solid is viewed in a phase contrast microscope and (iv) sharp peaks in the powder X-ray diffraction pattern. 

IR and Raman are sensitive to the rotation and vibration of molecules in solid phases (crystalline or x-ray amorphous). Molecular units of similar structure and composition absorb IR radiation in the same energy range, usually independent of the larger structure of the material this property makes IR spectroscopy useful for studying molecules in the interfacial region such as surface hydroxyl groups and As oxoanions on mineral surfaces, and for fingerprinting the local environment of As in aystalline... 

Liquid crystalline (LC) solutions of cellulose derivatives form chiral nematic (cholesteric) phases. Chiral nematic phases are formed when optically active molecules are incorporated into the nematic state. A fingerprint texture is generally observed under crossed polarizers for chiral nematic liquid crystals when the axis of the helicoidal structure is perpendicular to the incident light (Fig. 2). 

The major area of application for solids and liquids is chemical fingerprinting and the identification of unknown compounds. For solids, Raman is also used for phase identification, following amorphous/crystalline transitions, measurement of stress and strain, and, in the microscope mode, the detection and analysis of defects, including particles during wafer processing. 

Phase Analysis. If a specimen is crystalline, an obvious way to study and identify it is by using X-ray diffraction patterns. The X-ray diffraction pattern of a crystalline compound functions as a fingerprint that should permit unambiguous identification. Isomorphous crystalline compounds often create major problems elemental analysis can be of aid here. 

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