Identification of crystalline materials

Crystals produce different diffraction patterns when subjected to bombardment of monochromatic X-ray sources and thereby provide unequivocal identification of crystalline materials. 

Although x-ray diffraction has been used for many years in the identification of crystalline materials, its special application to the study of ceramic temper and paste minerals is relatively recent. Because many tem-... 

International Centre for Diffraction Data, (ICDD ) is a non-profit scientific organization dedicated to collecting, editing, publishing, and distributing powder diffraction data for the identification of crystalline materials. ICDD on the Web http //www.icdd.com/. 

XRD is used for the identification of crystalline materials such as pigments, metal powders, organic materials and salts. Non-crystalline materials lacking a regular crystal lattice, such as glass, do not produce a clear pattern. 

International Centre for Diffraction Data — Maintains and distributes the Powder Diffraction File (PDF), a file of over 500,000 X-ray powder diffraction patterns used for identification of crystalline materials. The ICDD also distributes the NIST Crystal Data file, which contains lattice parameters for over 235,000 inorganic, organic, metal, and mineral crystalline materials. Address 12 Campus Blvd., Newton Square, PA 19073-3273 [www.icdd.com]. 

Crystalline materials, in which the atomic spacing is about the same magnitude as x-ray wavelengths, are capable of diffracting x-rays. This serves as the basis of x-ray diffraction analysis qualitative identification of crystalline materials is readily made from a measurement of the angles of diffraction. X-ray diffraction also serves as a means for isolating x-rays of a particular wavelength in an x-ray spectrometer. 

The identification of crystalline materials can be determined by rapid computerized powder diffraction techniques. The principle of this technique [7] is that the crystallites within a sample, placed in a collimated x-ray beam, reflect x-rays at specific angles and intensities. The diffraction pattern can be recorded photographically, using a camera, e.g. a Debye-Scherrer camera, or using a powder diffractometer. Chemical analysis depends on the fact that each chemical composition and crystallographic structure produces a unique angular distribution of diffracted intensity. Analysis is based on comparison of the diffractometer scan with known standards. Typical applications of the powder diffraction technique to polymers would be the identification of mineral fillers in engineering resins, the nature of crystalline contaminants and determination of crystalline phases in a material. 

X-ray diffraction uses X-rays of known wavelengths to determine the lattice spacing in crystalline structures and therefore directly identify chemical compounds. This is in contrast to the other X-ray methods discussed in this chapter (XRF, electron microprobe analysis, PIXE) which determine concentrations of constituent elements in artifacts. Powder XRD, the simplest of the range of XRD methods, is the most widely applied method for structural identification of inorganic materials, and, in some cases, can also provide information about mechanical and thermal treatments during artifact manufacture. Cullity (1978) provides a detailed account of the method. 

Recent developments and prospects of these methods have been discussed in a chapter by Schneider et al. (2001). It was underlined that these methods are widely applied for the characterization of crystalline materials (phase identification, quantitative analysis, determination of structure imperfections, crystal structure determination and analysis of 3D microstructural properties). Phase identification was traditionally based on a comparison of observed data with interplanar spacings and relative intensities (d and T) listed for crystalline materials. More recent search-match procedures, based on digitized patterns, and Powder Diffraction File (International Centre for Diffraction Data, USA.) containing powder data for hundreds of thousands substances may result in a fast efficient qualitative analysis. The determination of the amounts of different phases present in a multi-component sample (quantitative analysis) is based on the so-called Rietveld method. Procedures for pattern indexing, structure solution and refinement of structure model are based on the same method. 

The primary source of x-ray crystal diffract-tion reference data is the above-mentioned ASTM Powder Diffraction File , published by the Joint Committee on Powder Diffraction Standards. This file consists of over 38000 diffraction patterns of crystalline materials including expls and related materials. Scientists in the expls field routinely utilize this source. for the identification of expls and metastable phases by comparing the interplanar d spacings and intensities of exptl phases with those of known phases (Refs 4,10,21 22)... 

The simplicity and advantages of the routine application of powder diffraction techniques for the chemical analysis and identification of polycrystalline materials were pointed out by Hull [9], who stated "that every crystalline substance gives a pattern that the same substance always gives the same pattern," and by the pioneering work of Hanawalt et al. [10]. 

There are two broad applications of X-rays in the characterization of materials (i) X-ray spectrometry and (ii) X-ray diffractometry. The former technique is used for chemical analysis and has found only limited use in the characterization of pharmaceuticals. On the other hand, X-ray diffractometry, by providing a means for the study of the structure of crystalline materials, is extensively used to characterize pharmaceutical solids. There are two principal applications of X-ray diffractometry. X-ray crystallography is concerned with the structure determination of crystalline phases. Single crystals are usually used for this purpose. On the other hand, in X-ray powder diffractometry, the sample is usually in the form of a powder. X-ray powder diffractometry is recognized as a powerful technique for the identification of crystalline phases. The technique can also be used for the quantitative analyses of solids. This article will be restricted to the principles and applications of X-ray powder diffractometry (XRD) in the characterization of pharmaceutical solids. 

Petrographic examination under the microscope can provide Identification of the material, and can be complemented with techniques such as x-ray diffraction (XRD) to Identify crystalline minerals. The quantitative measure of the relative amounts of the minerals Is a somewhat controversial subject due to some of the limitations of the techniques. However the comparison of the results from a number of laboratories has led to the conclusion that no one XRD method Is superior to all others. Repeated attempts to compare results between different laboratories has led to the consensus that the technique Is seml-quantItative. 

X-rays are electromagnetic radiation of wavelength about lA (10 m), which is about the same size as an atom. They occur in that portion of the electromagnetic spectrum between gamma rays and the ultraviolet. The discovery of X-rays in 1895 enabled scientists to probe crystalline structure at the atomic level. X-ray diffraction has been in use in two main areas for the fingerprint characterization of crystalline materials and the determination of their structure. Each crystalline solid has its unique characteristic X-ray powder pattern, which may be used as a "fingerprint" for its identification. Once the material has been identified. X-ray... 

X-ray or electron diffraction allows Identification of crystalline species by the long-used Hannawalt-Dow-ASTM-JCPDS system(4). Small particles can be removed for analysis in a small rotating specimen X-ray powder camera, or by extraction replication and selected area diffraction in a Transmission Electron Microscope (5). For those specimens where a residue of reactant or corrosion product is too adherent, the material may be removed for analysis by micro-bulldozing (with a microhardness Indentor), micro-jack hammering (with a needle attached to a small piezoelectric crystal on a pencil-like rod), and micro-boring (with a precision controlled dental drill)(5). 

X-rays have wavelengths in the range 0.1-100 A. The wavelengths used for crystallographic studies are typically in the range 0.5-2.5 A. Because of the availability and properties X-ray diffraction is the most common technique for phase identification and structural characterization of crystalline materials. X-rays are scattered by the electrons of the atoms. Important consequences of this are that ... 

Despite this, differences can still be observed allowing identification of amorphous material relative to the crystalline forms. 

Raw X-ray diffraction data, either digital or acquired on a strip recorder, are used make mineral identifications as summarized above. Whole sample data collected from random powder mounts are compared to patterns of known minerals either manually or using a computer search-match program such as /zPDSM (Marquart, 1986). Because each component in a mixture of crystalline materials produces its own characteristic pattern that is independent of others, the identification process becomes one of simply unscrambling the superposed patterns. 

X-ray Powdar Diffraction. X-ray powder diffraction (XRD) is a routine technique for the identification of crystalline phases present in a catalyst material (17), which routinely utilizes the comparing of an observed set of reflections from the catalyst sample with those of pure reference phases, or with a database. These XRD studies can now be carried out under in situ conditions on a working catalyst, in particular with the use of synchrotron radiation, which allows for the acquisition of data in real time (18). Time-resolved studies on the time scale... 

X-ray analysis methods (including diffraction and reflectometry) described in Chap. 1 are the most widely used tools for the identification of crystalline properties of materials, in addition to materials strain, texture, stress, density, and surface roughness—properties that are key parameters for various industrial applications. Chapter 2 covers a wide range of optical characterization techniques with focus on ellipsometry, Raman scattering, Fourier transform infrared spectroscopy, and spectrophotometry. Those methods, covering a wide range of photon energy and laser... 

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