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, p. 123

Section 3.3.6. Atomicity, chemistry and structure

B. H. Tobya*

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

3.3.6. Atomicity, chemistry and structure

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The structural model of a compound determined by powder-diffraction methods can be described by the data items in the core CIF dictionary. However, for a powder-diffraction study of a mixture of phases, the PD_PHASE category is used to list the phases present. This is the only category in the pdCIF dictionary that extends the description of the structural model beyond that covered by items in the core CIF dictionary. Table of phases

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The data items in this category are as follows:

PD_PHASE [Scheme scheme21]

The bullet ([\bullet]) indicates a category key.

When a sample contains more than one phase, the PD_PHASE data items are used to create a table describing the phases present. For example, the name and abundance of each phase can be specified using _pd_phase_name and _pd_phase_mass_%, respectively.

Two types of pointers can also be defined:

(i) Since the crystallographic description of each phase must be incorporated in a separate data block, _pd_phase_block_id contains the unique block ID (see Section 3.3.7[link]) pointing to the block containing the data for the phase.

(ii) An arbitrary label is assigned to every phase using _pd_phase_id so that reflections can be assigned to a phase using _pd_refln_phase_id. This is discussed further in Section[link].

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