International
Tables for Crystallography Volume G Definition and exchange of crystallographic data Edited by S. R. Hall and B. McMahon © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. G, ch. 3.3, p. 129
Section 3.3.9.4. Multiple sets of measurements and phases^{a}NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562, USA |
Multiple data sets may be used for mixtures as well as single phases. This is becoming increasingly common as more complex materials are studied using powder diffraction. The treatment of this case follows logically from that of Sections 3.3.9.2 and 3.3.9.3. If there are M diffraction data sets and P phases, there will be P blocks containing the crystallographic parameters for each phase. There will be M blocks with the observed and calculated diffraction intensities, as well as reflection tables. Depending on the Rietveld software, there may be M × P sets of some parameters, for example phase fractions and profile descriptions. These parameters may be placed in the phase table loop within the data-set block(s).
Ideally, the same specimen, or at least the same sample, will be used for all measurements. Sometimes, however, different samples are used for combined refinements to extend the number of observations, despite the possibility that the samples might have slightly different structures or compositions. If there are S samples, there will be an additional S blocks that record the sample and specimen preparation and characterization information. Thus, in this case there will be a total of M + P + S blocks.
As before, the phase blocks will use the block pointers _pd_block_diffractogram_id to link to the data-set blocks. Likewise, the data-set blocks will have phase tables with _pd_phase_block_id values that link to the phase blocks. The sample blocks can use both _pd_block_diffractogram_id values and _pd_phase_block_id values to link to the the diffraction data and the analysis results. This is shown in the CIF in Example 3.3.7.1. The program GSAS2CIF (Toby et al., 2003) can create CIFs for multiple sets of measurements and phases.
References
Toby, B. H., Von Dreele, R. B. & Larson, A. C. (2003). CIF applications. XIV. Reporting of Rietveld results using pdCIF: GSAS2CIF. J. Appl. Cryst. 36, 1290–1294.