Section 3.4.3.1. Experimental measurements

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:

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

The data items in these categories are as follows:

(a) CELL

(b) CELL_SUBSYSTEM

(and _cell_subsystem_matrix_W_m_n for all combinations )

(c) CELL_SUBSYSTEMS

(d) CELL_WAVE_VECTOR

(e) CELL_WAVE_VECTORS

As explained in Section 3.4.2, 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). _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). 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.

The categories describing data collection are as follows:

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

New data items in these categories are as follows:

(a) DIFFRN_REFLN

(b) DIFFRN_REFLNS

(c) DIFFRN_STANDARD_REFLN

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 indexing the modulation wave vectors when the diffraction wave vector is written as .

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 . _diffrn_standard_refln_index_m_* items allow the higher-dimensional Miller indices of standard reflections to be recorded.

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

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

New data items in these categories are as follows:

(a) EXPTL_CRYSTAL

(b) EXPTL_CRYSTAL_FACE

_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.