Classification and use of powder diffraction data

Abstract

Powder diffraction creates new demands for versatility in CIF. Unlike single-crystal experiments, where raw data can be automatically reduced to an instrument-independent form (structure factors), powder-diffraction data will always be instrument dependent, and there is a wide array of instrumentation: (i) X-ray: conventional, synchrotron, energy-dispersive; (ii) neutron: CW, TOF; (iii) and detection: single, multiple, linear position-sensitive, area, film. pdCIF has other special needs: (i) multiple crystallographic phases may be present (requires inter-block links); (ii) multiple sets of data may be used for a single model (inter-block links); (iii) and calibration data may appear in other CIFs (inter-file links). Since powder diffraction may be used for noncrystallographic purposes, such as phase identification, the data themselves may be the result. For classification purposes, the pdCIF dictionary is divided in sections. Few pdCIFs will have entries defined for all sections. The use of each section and the component data items is presented on a section-by-section basis, describing: the experimental configuration; sample preparation, characterization and mounting details; references to calibration information; the experimental (raw) diffraction data set; the processed diffraction data set; calculated diffractogram; peak table; reflection assignments and intensities; and derived structural results. The use of the different data items is shown through examples. This chapter is also available as HTML from the International Tables Online site hosted by the IUCr.