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. 122

Section 3.3.5.3. Diffraction peak table

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

3.3.5.3. Diffraction peak table

| top | pdf |

The data items in these categories are as follows:

(a) PD_PEAK [Scheme scheme18]

(b) PD_PEAK_METHOD [Scheme scheme19]

The bullet ([\bullet]) indicates a category key. The arrow ([\rightarrow]) is a reference to a parent data item. Items in italics are defined in the core CIF dictionary.

When diffraction intensities are first measured, particularly when attempting to identify unknown phases in a material, the first step in the analysis is often to compile a list of peak positions. These peak positions are commonly used to search the Powder Diffraction File, which contains lists of peak heights and positions for approximately 100 000 materials (International Centre for Diffraction Data, 2004[link]).

Information on diffraction peaks is recorded in the PD_PEAK section of the pdCIF. Peak positions are recorded using _pd_peak_2theta_maximum or _pd_peak_2theta_centroid, for positions determined from the intensity maxima or from the peak centroids, respectively. It is also possible to record peak positions using _pd_peak_d_spacing. Peak intensities are recorded using _pd_peak_intensity and _pd_peak_pk_height, for the integrated peak area or the intensity value at the peak maximum, respectively. Peak widths are recorded using _pd_peak_width_2theta and _pd_peak_width_d_spacing.

A separate loop is used to list reflections, as will be discussed in Section 3.3.5.4[link]. To link reflections to peaks (one peak may consist of many reflections), each peak is assigned a unique code using _pd_peak_id, which is then referenced in the reflection table using _pd_refln_peak_id.

When intensities are measured using radiation with more than one wavelength, for example when both Cu Kα1 and Kα2 radiation are used or when a monochromator passes both [\lambda] and [\lambda/2] radiation, peaks may be assigned a wavelength symbol using _pd_peak_wavelength_id, where the wavelength symbol is defined in a separate _diffrn_radiation_wavelength_id loop. However, for many experiments, the assignment of wavelengths to peaks will be impractical owing to reflection overlap. It is usually better practice to specify wavelength labels in the reflection table using _pd_refln_wavelength_id.

References

International Centre for Diffraction Data (2004). Powder Diffraction File. International Centre for Diffraction Data, 12 Campus Boulevard, Newtown Square, Pennsylvania, USA. http://www.icdd.com .








































to end of page
to top of page