Search Powder Diffraction

Phase identification using powder diffraction data requires a comparison of several key features present in its digitized pattern with known compounds/phases. This is usually achieved by searching powder diffraction database(s) for records, which match experimentally measured and digitized pattern. Thus, a powder diffraction database or at least its subset should be available in addition to a suitable search-and-match algorithm. 

Sea.rch-Ma.tch. The computer identifies which crystalline phases (components) match the unknown pattern by using a file of known powder patterns maintained by the International Center for Diffraction Data (ICDD). The Powder Diffraction File contains interplanar t5 -spacings d = A/(2sin0)] and intensities of the diffraction maxima for each crystalline powder pattern submitted to the ICDD. Currendy there are about 65,000 patterns in the file. Current search—match programs can successfully identify up to seven components in an unknown pattern. A typical diffraction pattern of an unknown sample and the components identified by the computer search-match program is shown in Figure 15. 

Materials Characterization. Regarding education in the characterization or analysis of materials—a central topic of materials chemistry—there is a similar hierarchy of importance of subjects that chemistry students (and faculty) will need to have learned. Reference 7 treats this topic systematically, and Roy and Newnham (11) presented a comprehensive (albeit somewhat outdated) presentation of the architecture of materials characterization. Thus Rutherford backscattering and extended X-ray absorption fine structure (EXAFS) are excellent characterization research tools, but in the sequence of tools used every day on every sample, they are insignificant. Thus for structural characterization, X-ray powder diffraction reigns supreme, yet the full power of the modern automated search routines that can be universally applied are taught only to a minuscule fraction of even the materials science student body. 

Powder Diffraction File, Inorganic Phases, Organic and Organometallic Phases Search Manual, W. F. McClune, editor-in-chief, International Centre for Diffraction Data, Newtown Square, Pa., 1993. 

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. 

F. X-ray Powder Difi action Search System. Compounds that fail to crystallize may still be examined by X-ray diffraction, because non-crystalline materials, as powders, give characteristic diffraction patterns. A collection of powder diffraction patterns proves to be a very effective means by which to identify materials and indeed, one of the very earliest search systems in chemical analysis was based upon such data by Hanawalt (21) over forty years ago. The importance of these data in TSCA can be seen by examining the TSCA Inventory regulations for treatment of confidential chemicals (22). Section 710.7 of these regulations indicates that EPA intends to rely on powder diffraction data to assure the validity and seriousness of a manufacturers request for treating information on a chemical as confidential. 

The data base of some 27,000 powder diffraction patterns that is used in the CIS (5) is in fact a direct descendant of that with which Hanawalt carried out his pioneering work. A problem that arises in connection with this particular component stems from the fact that powders, as opposed to crystals, are frequently impure and so the patterns that are obtained experimentally are often combinations of one or more file entries. A reverse searching program, that examines the experimental data to see if each entry from the file is contained in it, has been written after the general approach of Abramson (23), and seems to cope with this particular difficulty. It is currently running in test on the NIH PDP-10 and will be made available to the scientific community during the latter part of 1978. 

Because of the number of standards in the PDF, it is not possible, in general, to search each entry individually. However, in many situations the analyst either knows or can make an educated guess as to the probable identity of the unknown. In this intuitive case, the PDF can be consulted directly and the cards for the suspected compounds can be consulted quickly. In fact, when a specific phase diagram is being investigated it is common practice to obtain powder diffraction patterns of all known phases in the system. If the 20 axis of these "reference" patterns is the same as that for unknown patterns, an unknown pattern can be compared directly to these relatively few standards on an illuminated viewing table and the diffraction lines from the known phases can be identified on the unknown pattern. As new phases are discovered and identified, their patterns may then be added to this reference file. 

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 data may be helpful but are often hard to interpret for complex mixtures use of computer data file search programs (6) and microcamera methods for single particle analysis (7) may be useful for identification. Comparative sample identification is generally less often possible than for metals since the latter are manufactured while the nonmetallic inorganic solids are often unprocessed materials with large property variations. However, where applicable, the following are some examples of determinations which might be made (a) particle size by microscopy (b) microstructure and sub-microstructure characterization... 

In addition to structure determination from powder diffraction data as described earlier, another area of considerable current interest is the computational prediction of crystal structures based on energy simulation techniques. In such work, the potential energy, E(r), is computed as a function of the set of variables T that define the structure (the unit cell and space group are usually also included as variables in such calculations), and the E r) hypersurface is searched to find the structure of minimum energy. Representative examples of work in this field may be found in Refs. [74-79], and some work involving the use of evolutionary algorithms to carry out the search procedure has been reported [80-82]. 

Generally, an area detector permits one to check the sample quality much faster and more conclusively, because of easier visualization. Thus if a supposed single crystal gives a set of powder diffraction rings or an amorphous-state halo, the operator will see this immediately it would take a lot of time and expertise to extract this from the results of a random search ... 

Data collection software is generally supplied by the instrument manufacturers. This data collection software would also normally be equipped to perform preliminary analysis including peak finding routines and search/match of powder diffraction patterns from a database. The most popular database for powder diffraction data is maintained by the International Centre for Diffraction Data. ... 

The crystal used for data coUection was transferred to an Enraf-Nonius CAD-4 diffractometer. Automatic peak search and indexing procedures yielded the same monoclinic cell as derived from the X-ray powder diffraction data and precession photographs. Testing showed that the cell was indeed primitive and that there was no superlattice present. Table 5 gives the crystal data and X-ray experimental parameters, and Table 6, the interatomic distances and angles. Positional and thermal parameters are given in Table S3 (Supporting Information)... 

Powder Diffraction File Search Manual, Alphabetical Listing and Search Section of Frequently Encountered Phases, Inorganic Compounds. L.G.Berry et.al.(EdsO,Swarthmore, USA, (1976)... 

Smoothing is a numerical conditioning procedure employed to suppress statistical noise, which is present in any powder diffraction pattern as a result of random intensity measurement errors (Eq. 3.8 in Chapter 3). It improves the appearance of the powder diffraction pattern. For example, smoothing can make quickly collected data (say in a 15 min experiment) look similar to a pattern collected in a longer (e.g. in an overnight) experiment, and may help with certain automatic procedures, such as background subtraction, Ka2 stripping and unbiased peak search. 

The presence of dual wavelengths in conventional x-ray sources, or in other words the presence of the Ka2 component in both the incident and diffracted beams, complicates powder diffraction patterns by adding a second set of reflections from every reciprocal lattice point. They are located at slightly different Bragg angles when compared with those of the main (Kai) component. This decreases resolution and increases overlapping of Bragg peaks, both of which have adverse effect on an unbiased peak search. 

Recently the ICDD Powder Diffraction File underwent a substantial and useful upgrade calculated patterns based on single crystal data from the ICSD file have been included into the PDF-2/PDF-4 Full File calculated patterns of structures stored in the CSD file, have been included into the PDF-4 Organics (see Table 4.3). These additions make it possible to conduct searches and find matches with computed digitized powder patterns in addition to experimentally measured powder diffraction data, thus improving automation, simplifying phase identification process and considerably expanding the applicability of the powder method for a qualitative phase analysis. 

It makes little sense to search organic and metal organic structures database if the material from which powder diffraction data were collected is inorganic, and vice versa. 

Overall, phase identification in a multiple-phase material, which consist of more than two phases is difficult and often has no reasonable solution in a blind search, especially when none of the phases have been positively identified prior to the search using a different experimental technique. Furthermore, chances for success decrease proportionally to the increased complexity of the measured powder diffraction pattern, unless the number of possible components with different crystal structures in the mixture is limited to just a few. 

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