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For Submission to the International Union of Crystallography Commission on Powder Diffraction Newsletter.

Using CIF for Powder Diffraction

Brian Toby

Most crystallographers are aware of the IUCr's efforts to define a standardized format for crystallographic information known as the Crystallographic Information File (CIF). Those crystallographers who are not may wish to review the document "A Very Brief Introduction to CIF" or consult the enclosed references. The initial definitions developed for CIF were designed to suit the needs of the small-molecule single-crystal crystallographic community. However, the importance of efforts to broaden the applicability of CIF to suit the needs of the macromolecular and the powder diffraction community were recognized early.

Over the last few years, Syd Hall, Ian Langford, and I, with the assistance of many people, have been working on expanding the definitions in CIF for powder diffraction applications. These efforts have been assisted by the members of the Commission on Powder Diffraction (CPD) and the Commission on CIF (COMCIFS). A draft of these additions have been completed in the form of a supplementary dictionary for powder diffraction. The draft dictionary is available to the crystallographic community for review prior to its adoption as a standard (see "On-line Access to Information About CIF.") Those crystallographers who do not have access to electronic mail may contact the Research and Development Officer of the IUCr, Brian McMahon, for printed information.

The assistance of the powder diffraction community is needed to ensure that definitions used in the dictionary conform to proper usage and are sufficiently comprehensive to include the vast majority of powder diffraction applications. It should be noted that powder diffraction CIF files will very likely use definitions in both the original (core) CIF dictionary and the powder CIF dictionary, so both dictionaries should be reviewed together. Comments about the draft dictionary may be addressed to the chair of COMCIFS, I. David Brown (IDBrown@mcmaster.ca) or Brian McMahon. Questions may be addressed to the principal author of the powder dictionary, Brian Toby (tobybh@ttown.apci.com).

While the goal of the initial CIF effort was to define a file format to contain information typically found in a single-crystal structure manuscript (and supplementary materials), the powder CIF effort had the additional goals of documenting and archiving experimental data. It is intended that powder CIF be used for communication of completed studies, but also for data exchange between laboratories. This is frequently done at shared diffraction facilities such as neutron and synchrotron sources. The powder dictionary was written anticipating needs from conventional x-ray diffractometers, as well as from synchrotron, CW neutron, TOF neutron, and energy dispersive x-ray instruments. Due to my lack of familiarity, a more cursory attempt was made for film instruments. We have delayed addressing requirements for image plates and area detector instruments until issues relating to storing 2-D arrays in CIF are resolved.

A powder CIF may contain descriptions of:

A powder CIF can also document diffraction data where multiple phases are indexed or refined. It is also possible to document a Rietveld refinement that incorporates multiple datasets.There is no requirement that powder CIFs include all of the above information. Indeed no powder CIF will ever utilize all of the data items defined in the powder dictionary. Data items used in CIF are described in a dictionary, which defines meaning and usage. For example, the dictionary entry defining _cell_length_a specifies that the value will be a number in angstroms and that an ESD is allowed. The dictionary is itself a STAR file, where the dictionary syntax is defined in a separate Dictionary Definition Language (DDL) file.

References:

CIF: S. Hall, F. Allen & I. D. Brown, Acta Cryst. A 47 (1991), 655-685.

STAR: S. Hall & N. Spadaccini, J. Chem. Inf. Comput. Sci. 34 (1994), 505-508.


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