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. 3.4, pp. 133-134

Section 3.4.3.1. Experimental measurements

G. Madariagaa*

aDepartamento de Física de la Materia Condensada, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, 48080 Bilbao, Spain
Correspondence e-mail: gotzon.madariaga@ehu.es

3.4.3.1. Experimental measurements

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The categories relevant to a structure determination experiment are in the CELL, DIFFRN and EXPTL groups. Most of the items extend the existing core CIF categories, but the categories describing cell subsystems are new.

3.4.3.1.1. Cell and modulation wave vectors

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The categories describing the unit cell (or cells for composite structures), the wave vectors of the modulations and, for composites, the cell subsystems are as follows:

CELL group
CELL
CELL_SUBSYSTEM
CELL_SUBSYSTEMS
CELL_WAVE_VECTOR
CELL_WAVE_VECTORS

Categories marked with ¶ are already defined in the core CIF dictionary.

The data items in these categories are as follows:

(a) CELL [Scheme scheme1]

(b) CELL_SUBSYSTEM [Scheme scheme2] [\vdots]

(and _cell_subsystem_matrix_W_m_n for all combinations [1 \le m,n \le 11])

[\vdots] [Scheme scheme3]

(c) CELL_SUBSYSTEMS [Scheme scheme4]

(d) CELL_WAVE_VECTOR [Scheme scheme5]

(e) CELL_WAVE_VECTORS [Scheme scheme6]

As explained in Section 3.4.2[link], the msCIF dictionary arbitrarily allows an upper limit of 11 for the dimension of superspace for which data names are defined. _cell_modulation_dimension specifies the number of additional reciprocal vectors needed to index the whole diffraction pattern and has values d ranging from 1 to 8 to express the dimensionality (3 + d) of the superspace. _cell_reciprocal_basis_description is a text field allowing a free description of the higher-dimensional basis chosen.

For a composite structure, different cell subsystems may be specified. Each such subsystem is identified and characterized by the data items in the CELL_SUBSYSTEM category (see Section 3.4.4.1[link]). _cell_subsystems_number gives the number of such subsystems as an independent check of the completeness of the description.

Data items in the CELL_WAVE_VECTOR category specify the wave vectors of the modulation (see Section 3.4.4.1[link]). In accordance with the limits on dimensionality of the current version of the msCIF dictionary, no more than eight additional modulation wave vectors may be specified. The number used must agree with the value of _cell_modulation_dimension.

The data items in the CELL_WAVE_VECTORS category describe the experimental conditions during the determination of the independent modulation wave vectors.

3.4.3.1.2. Data collection

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The categories describing data collection are as follows:

DIFFRN group
DIFFRN_REFLN
DIFFRN_REFLNS
DIFFRN_STANDARD_REFLN

Categories marked with ¶ are already defined in the core CIF dictionary.

New data items in these categories are as follows:

(a) DIFFRN_REFLN [Scheme scheme7]

(b) DIFFRN_REFLNS [Scheme scheme8]

(c) DIFFRN_STANDARD_REFLN [Scheme scheme9]

The data items in these categories are straightforward extensions of the core CIF dictionary definitions to the indexing of diffraction intensities by higher-dimensional components. The _diffrn_refln_index_m_* items are the additional Miller indices [m_i] indexing the modulation wave vectors [{\bf q}_i] when the diffraction wave vector is written as [{\bf H}=h {\bf a}^{*}+k {\bf b}^{*}+ l{\bf c}^{*}+ \textstyle\sum_i m_{i} {\bf q}_{i}].

The upper limit of 8 was chosen arbitrarily to limit the number of data items defined in the msCIF dictionary.

_diffrn_reflns_limit_index_m_* items provide independent checks on the range of values recorded for each of the additional Miller indices [m_i]. _diffrn_standard_refln_index_m_* items allow the higher-dimensional Miller indices of standard reflections to be recorded.

3.4.3.1.3. Experimental measurements on the crystal

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The categories describing measurements on the crystal or crystals used in the experiment are as follows:

EXPTL group
EXPTL_CRYSTAL
EXPTL_CRYSTAL_FACE

Categories marked with ¶ are already defined in the core CIF dictionary.

New data items in these categories are as follows:

(a) EXPTL_CRYSTAL [Scheme scheme10]

(b) EXPTL_CRYSTAL_FACE [Scheme scheme11]

_exptl_crystal_type_of_structure specifies the structure type as cryst (crystalline), mod (modulated) or comp (composite). These are the only three types of structure handled at present by the msCIF dictionary.

The extensions to the EXPTL_CRYSTAL_FACE category permit the indexing of crystal faces using the higher-dimensional Miller indices introduced for aperiodic structures.








































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