International
Tables for
Crystallography
Volume G
Definition and exchange of crystallographic data
Edited by S. R. Hall and B. McMahon

International Tables for Crystallography (2006). Vol. G, ch. 3.3, p. 117

Section 3.3.2. Dictionary design considerations

B. H. Tobya*

aNIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562, USA
Correspondence e-mail: brian.toby@nist.gov

3.3.2. Dictionary design considerations

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The many applications of powder diffraction and the wide range of different instruments used dictate the design of several aspects of the powder diffraction CIF (pdCIF) data dictionary. In contrast to single-crystal measurements, where observations are reduced to instrument-independent structure factors, in powder diffraction it is not possible to interpret the data without a detailed knowledge of the type of instrument used. Another difference is that while single-crystal measurements are rarely made for any purpose other than structure determination, powder diffraction data have many non-crystallographic applications (e.g. identifying the phases in a mixture of unknown composition). In a single-crystal diffraction experiment, one sample yields one crystal structure. In a powder diffraction experiment, however, it is common for the sample to be a mixture of phases. Analysis of the diffraction data may result in multiple crystal structure models. It is also common to use multiple data sets, for example to fit one crystallographic model to both X-ray and neutron data simultaneously.

The powder dictionary was developed with the following basic objectives in mind. A pdCIF:

(i) should record the data-collection experiment as completely as possible and document any data analysis;

(ii) should be appropriate for the exchange of unprocessed measurements, so that it can be used in national laboratories and other shared facilities;

(iii) may contain more than one data set and/or crystal structure through the use of multiple data blocks;

(iv) may accommodate references between data blocks; and

(v) should recognize and accommodate data from as many different types of instruments as possible.

The CIF syntax, as opposed to the STAR File syntax, is not well suited to handling large multi-dimensional data sets. For some two-dimensional image formats, this deficiency was addressed by the development of imgCIF (see Chapters 2.3[link] and 3.7[link] ). It is also true that the CIF syntax is not well suited to storing unprocessed powder-diffraction measurements from the many instruments that use area detectors (particularly for the case of the three-dimensional data structures needed for modern TOF instruments). Even in these cases, however, diffraction intensities are commonly reduced to simpler representations, such as might be input to a Rietveld refinement program. The pdCIF definitions are intended for use with these reduced data sets.








































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