Section 3.3.7.1. Use of block pointers

More complex link structures will be needed when multiple data sets and multiple phases occur together. Example 3.3.7.1 outlines a pdCIF reporting the results of a TOF powder-diffraction study of a physical mixture of nickel and silicon powders in which two separate diffraction banks, measured at two different Bragg angles, were used. In this case, five CIF blocks are used. The first CIF block reports the overall and publication details. The next two CIF blocks report crystallographic information for each phase and the last two blocks report the observed, processed and calculated diffraction intensities and reflection tables.

A second purpose for _pd_block_id is to provide a mechanism for tracking successive modifications to a CIF. Consider the case where a data set is obtained at a user facility and the resulting measurements are distributed as a CIF. In this file, a value is supplied for _pd_block_id based on the time when the measurements were made. At a later time, when these observations are analysed, a new CIF is created, containing both the original measurements and the results from the analysis. Rather than replace the original value for _pd_block_id, the data item can be placed in a loop and another value, defining a second block ID, can be added. This will indicate the connection to the initial CIF, since the original block ID is retained.

A potential future use for block pointers may be to reference non-CIF data files that contain large two- and three-dimensional data structures. This is expected to become increasingly important as neutron and synchrotron instruments are constructed that cover increasing ranges of solid angle. As mentioned in Section 3.3.2, CIF is not well suited to these complex, large and possibly irregular measurement arrays. The NeXus format has been developed by a consortium of synchrotron and neutron laboratories to address these concerns and is currently being used for a variety of scattering applications (NeXus, 1999). The NeXus format is based on the platform-independent HDF binary standard (HDF, 1998). The use of block pointers to resolve references to non-CIF documents will require additional definitions.