International Centre for Diffraction Data ICDD

The International Centre for Diffraction Data (ICDD, Newtown Square, Pa.) maintains a collection of single-phase x-ray powder patterns [5]. There are separate listings of inorganic and organic compounds. The x-ray diffraction data of ibuprofen, as a representative example, is given here (Fig. 1). 

International Centre for Diffraction Data (ICDD) (2006). Powder diffraction file Organic and organometallic phases search manual for experimental patterns. International Centre for Diffraction Data, Newton Square, PA. 

X-ray powder diffraction files (PDFs) for explosive materials are available as subsets of PDF libraries, such as Xpowder and that of the International Centre for Diffraction Data (ICDD). Commercial X-ray diffractometers can yield lattice spacings, d, to a precision of 0.0001 A. At this accuracy, it is not difficult to distinguish uniquely between many thousands of chemical compounds. Hence, XRD is a standard analytical technique for precisely identifying crystalline materials. 

The immense number of chemical compounds formed by the halogens provides chemists with an extraordinary database from which numerous chemical and physical phenomena can be correlated with respect to various periodic trends. From databases like Inorganic Crystal Structure Data (ICSD, http //www.fiz-karlsruhe.de ) and International Centre for Diffraction Data (ICDD, http //www.icdd.com) with 67 000 and 25 000 entries, respectively, one can easily make out that halides are one of the dominant classes of compounds besides oxides. Even within the subset of inorganic solids, there is tremendous diversity of composition, stracture, and properties and to summarize this would create its own encyclopedia. Therefore, the discussion in this article is limited primarily to binary halides, their structures, and some of their properties, except halides of elements which are nonmetals. Binary actinide hahdes are discnssed elsewhere see Actinides Inorganic Coordination Chemistry). Complex hahdes (sohd phases containing two or more kinds of metal ions), ... 

The simplest application of powder diffraction is to identify a crystalline phase. One merely runs the powder pattern of the solid and records either 29 values or rf-spacings or both. This information is given by the program and can be printed directly on the pattern. Then one may consult a database such as the JCPDS file. These initials refer to the JCPDS. As a result of the complexity of keeping pace with the ever growing fist of known phases, the operation grew and was renamed the International Centre for Diffraction Data (ICDD), headquartered in Newton Square, Pennsylvania. 

FIGURE 16.5. Data for two salts from the Powder Diffraction File. The crystal system, unit-cel] dimensions, space group and bibliographic references are given together with experimental details. Listed on the tight ate dhki, I hkl) and hkl for the most intense Bragg reflections. Courtesy, JCPDS — International Centre for Diffraction Data (ICDD), publishers of the Powder Diffraction File. 

The optimal counting time depends on the requirements imposed by the desired quality of diffraction data. For example, the International Centre for Diffraction Data (ICDD), which maintains and distributes the most extensive database of powder diffraction data, has established the following requirements for the submission of new experimental powder diffraction patterns to be added to the Powder Diffraction File ... 

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/. 

Identification of crystalline substance and crystalline phases in a specimen is achieved by comparing the specimen diffraction spectrum with spectra of known crystalline substances. X-ray diffraction data from a known substance are recorded as a powder diffractionfile (PDF). Most PDFs are obtained with CuKa radiation. Standard diffraction data have been published by the International Centre for Diffraction Data (ICDD), and they are updated and expanded from time to time. For one crystalline substance, there may be more than one file. The most recently updated file is recommended for phase identification. The early PDFs may contain errors in data obtained experimentally. More recently published PDFs are either obtained by more accurate experimental measurements or by theoretical calculation. A specimen to be identified should be in a powder form for most accurate matching. When we need to identify the crystal structure of a specimen that cannot be prepared as powder, matches of peak positions and relative intensities might be less than perfect. In this case, other information about the specimen such as chemical composition should be used to make a judgment. 

In modern powder diffraction the measurement delivers a raw-file of some thousand step-scan data of counted X-ray photons per step. This raw file contains all the needed information to carry out a crystallographic analysis, but in a way that requires follow up. More informative is a list of distinguishable reflections that includes the position (mostly in the form of f-values) and intensity of each reflection. This dif-file (d-values and intensities) contains some tens to hundreds of reflections. The number of reflections depends on the complexity of the structure and the crystal symmetry the more atoms per cell and the lower the symmetry the more reflections can be identified. But the number of detectible reflections also depends on the resolving power of the equipment, best documented by the half-width of the reflections (more accurately half-width at half-maximum, FWHM). Reflections nearer together than this half-width (or even two half-widths) cannot be resolved. In a second step, very often the Miller indices of the originating lattice planes are added to the dif-file. For this the knowledge of the unit cell is necessary (though not of the crystal structure itself). The powder diffraction file PDF of the International Centre for Diffraction Data (ICDD) contains over 100000 such dif-files for the identification and discrimination of solid state samples. 

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. 

International Centre for Diffraction Data- ICDD, Powder Diffraction File, Newtown Square, Pennsylvania, 1997. 

Every crystalline powder produces a characteristic diffraction pattern. This is the basis of qualitative analysis by powder diffraction. Identification is usually accomplished by systematic comparison of an unknown pattern with a catalog of standard data such as the Powder Diffraction File published by the International Centre for Diffraction Data (ICDD). Specialized techniques have been designed for certain situations. 

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