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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. 128
Section 3.3.9.1. A single phase
a
NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562, USA |
When a single set of diffraction measurements is used to model a single phase, a pdCIF will usually contain only one block. There will be several important loops present.
One loop will contain atomic parameters, such as coordinates. The unit cell must also be specified.
A second loop will contain the reflection table.
A third loop will contain the observed (or processed) diffraction measurements and the simulated pattern. Other items that should be included in this loop are the least-squares weights (usually σ−2, where σ is the standard uncertainty) so that it is possible to determine the quality of the fit in individual regions. Weight values of zero can also be used to indicate that data points have been excluded from the refinement. Since background fitting is quite important in Rietveld analysis, it is also valuable to include the background values. Thus, this loop should specify:
(i) the ordinate of the Rietveld plot, using one or more of: _pd_meas_2theta_scan, _pd_meas_time_of_flight, _pd_proc_2theta_corrected, _pd_proc_d_spacing or _pd_proc_recip_len_Q; alternatively the ordinate can be specified using either _pd_meas_2theta_range_* or _pd_proc_2theta_range_*, where _* is _min, _max and _inc outside the loop.
It is recommended that all CIFs describing the results of a Rietveld refinement include either _pd_proc_d_spacing or _pd_proc_recip_len_Q.
(ii) The observed (or processed) intensity values, using the items _pd_meas_counts_total, _pd_meas_intensity_total, _pd_proc_intensity_total or _pd_proc_intensity_net.
(iii) The background, using the item _pd_proc_intensity_bkg_calc.
(iv) The least-squares weights, using the item _pd_proc_ls_weight. If these weights are not specified, then it must be presumed that all points have been used in the refinement and that the weights are the reciprocal of the intensity values (if _pd_meas_counts_total was used) or the reciprocal of the intensity standard uncertainties, if specified.
(v) The calculated pattern should appear using either _pd_calc_intensity_net or _pd_calc_intensity_total.
It is good practice always to include at least one data item from each entry in the list above.
Apart from the information contained in these loops, information from almost all sections of the pdCIF dictionary can be valuable. Such items include data items that define how the diffraction measurements were made, how the sample was prepared and characterized, how the refinement was performed, and least-squares parameters and R factors. A template and an example pdCIF showing the combined use of pdCIF and core data items form part of the Acta Crystallographica instructions for authors at http://journals.iucr.org/services/cif/powder.html .
