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, pp. 122-123

Section 3.3.5.4. Reflection assignments and intensities

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

3.3.5.4. Reflection assignments and intensities

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In addition to the REFLN data items defined in the core CIF dictionary, the following items are defined:

REFLN [Scheme scheme20]

The arrow ([\rightarrow]) is a reference to a parent data item. The dagger ([\dagger]) indicates a deprecated item, which should not be used in the creation of new CIFs. Items in italics are defined in the core CIF dictionary.

In a single-crystal experiment, a reflection table contains the initial experimental observations for structural analysis. In contrast, the reflection table for a powder-diffraction experiment is a derived result that depends on the model used to apportion intensity between overlapping reflections. Another difference is that in a single-crystal experiment, the reflection list will refer to only one phase (one hopes), while it is common to have reflections from more than one phase in a powder-diffraction reflection list.

A list of reflections in a powder-diffraction pattern is commonly generated by Rietveld analysis, where Hugo Rietveld's algorithm (Rietveld, 1967[link], 1969[link]) is used to estimate the intensity of each reflection. Alternatively, when the structure of one or more phases is not known, it is possible to use full-pattern intensity-extraction methods such as the algorithms developed by Pawley (1981[link]) or Le Bail et al. (1988[link]). In fact, intensity information obtained by full-pattern intensity extraction is often used for ab initio structure determination.

Most of the information in the reflection table will be defined using data items from the core CIF dictionary (see Section 3.2.2.2[link] and Chapter 4.1[link] ). For example, _refln_index_h, _refln_index_k and _refln_index_l will be used for the indices. The structure factors and reflection intensities are specified using _refln_intensity_calc, _refln_intensity_meas, _refln_F_squared_calc and _refln_F_squared_meas; reflection positions are defined using _refln_d_spacing. To link a reflection with a powder-diffraction peak, the pdCIF data item _pd_refln_peak_id is used. The value for _pd_refln_peak_id serves as a pointer to an entry in the peak table which has been labelled, using the data name _pd_peak_id, with the same symbol. Likewise, to link a reflection to a phase, the pdCIF data item _pd_refln_phase_id points to a phase defined using _pd_phase_id in the phase table. Since a single reflection may be observed with more than one wavelength, for example, with [\lambda/2] or Kα2 wavelengths, the pdCIF dictionary defines a wavelength link, _pd_refln_wavelength_id, that defines a wavelength label. However, since version 2.1, the core CIF dictionary defines _refln_wavelength_id and this should be used in preference to _pd_refln_wavelength_id. The data items _refln_wavelength_id and _pd_refln_wavelength_id both point to a wavelength label defined using _diffrn_radiation_wavelength_id.

The International Centre for Diffraction Data abstracts peak positions and heights for inclusion in the Powder Diffraction File. This information would be found in the _pd_peak section of a pdCIF. However, in many studies, particularly in Rietveld refinements, peak tables are never generated. In principle, it should be possible to calculate peak positions and peak heights (or better still, peak areas) from the information in a reflection table. An algorithm for this would be very useful.

References

Le Bail, A., Duroy, H. & Fourquet, J. L. (1988). Ab initio structure determination of LiSbWO6 by X-ray powder diffraction. Mater. Res. Bull. 23, 447–452.
Pawley, G. S. (1981). Unit-cell refinement from powder diffraction scans. J. Appl. Cryst. 14, 357–361.
Rietveld, H. M. (1967). Line profiles of neutron powder-diffraction peaks for structure refinement. Acta Cryst. 22, 151–152.
Rietveld, H. M. (1969). A profile refinement method for nuclear and magnetic structures. J. Appl. Cryst. 2, 65–71.








































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