Tables for
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. 117-118

Section 3.3.3. pdCIF dictionary sections

B. H. Tobya*

aNIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562, USA
Correspondence e-mail:

3.3.3. pdCIF dictionary sections

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At the time when the powder CIF dictionary was created, it was recommended practice to give data items in other dictionaries names that distinguished them from items in the core CIF dictionary. This recommendation proved cumbersome and was subsequently withdrawn, but pdCIF data items are all named in a way that distinguishes them clearly from core CIF dictionary data items: data names in the pdCIF dictionary all begin with the string _pd_ and, likewise, the data categories in the pdCIF dictionary all begin with PD_.

The data items in the pdCIF dictionary are named according to the type of information they contain, rather than the way they will be used in a data structure. For example, data items relating to observed data have names beginning with _pd_meas_, even though some data items beginning with _pd_meas_ may be assigned to the same category as items in other parts of the dictionary. The formal category assignments (see Section[link] ) were dictated by the aim to make pdCIF as versatile and simple to use as possible. As an example, one might want to put both the observed and calculated intensities in a single table (loop), as would be the case for a Rietveld refinement. This requires that the calculated and observed intensity data items are assigned to the same category. Note that this does not require that these items always appear in a single loop; calculated and observed intensity values could appear in separate loops, for example, if the increment between data points differs.

This need to contain diverse items in a common `looped' list has led the pdCIF dictionary to use category names in a different way from the other CIF dictionaries, in which CIF data items are usually named according to their category. In the pdCIF dictionary, data items that might appear in the loop for diffraction intensities are assigned to the category PD_DATA. Only one data item is named using this category as prefix, _pd_data_point_id. Another departure from the convention used in other dictionaries is that several _pd_refln_* data names are assigned to the category REFLN so that these items may be included in a loop with _refln_* items defined in the core CIF dictionary.

Table[link] summarizes the category groups in the pdCIF dictionary; the individual categories are listed alphabetically in Appendix 3.3.1[link]. The appendix also lists for each category the section of this chapter in which the category is described.

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Category groups defined in the powder CIF dictionary

The groups are listed in the order in which they are described in this chapter.

SectionCategory groupSubject covered
(a) Experimental measurements[link] PD_CHAR Characterization of a sample[link] PD_PREP Preparation of a sample[link] PD_SPEC Specimen used in an experiment[link] PD_CALIB Calibration information[link] PD_INSTR The experimental instrument[link] PD_MEAS Raw measurements and instrumental settings
(b) Analysis[link] PD_PROC Processed settings[link] PD_CALC Simulated settings[link] PD_PEAK Diffraction peak table[link] REFLN Reflection assignments and intensities
(c) Atomicity, chemistry and structure[link] PD_PHASE Phases present
(d) File metadata[link] PD_BLOCK Relationships between data blocks[link],[link],[link],[link] PD_DATA Measured and simulated intensities

The order in which the categories are discussed follows the scheme of Table[link] , so that the contents of the dictionary are summarized under the headings Experimental measurements (Section 3.3.4[link]), Analysis (Section 3.3.5[link]), Atomicity, chemistry and structure (Section 3.3.6[link]) and File metadata (Section 3.3.7[link]). The pdCIF dictionary does not contribute any new data items relevant to publication beyond those already in the core CIF dictionary.

The data items in each category are listed below. Category keys, if specified, are listed first and are marked by a bullet ([\bullet]); the remaining data items in each category are listed alphabetically. Note that the category PD_DATA is discussed in several different sections.

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