Whole powder pattern decomposition methods and applications: A retrospection

TLDR: The genesis of the modern whole powder pattern decomposition methods (the so-called Pawley and Le Bail methods) is detailed and their importance and domains of application are decoded from the most cited papers citing them.

Abstract: A modern definition for whole powder pattern decomposition WPPD methods would be that they simultaneously have to refine the unit-cell parameters and extract the best estimations of the Bragg peak intensities from a complete diffractogram. This is done very fast nowadays, irrespective of the number of Bragg peaks present in a powder diffraction pattern, but we did not attain this comfortable situation without some past efforts. The WPPD methods’ introduction occurred slowly and progressively thanks to the increase in computer power, the improvements in graphical user interfaces, the diffractometer data digitalization, the availability of synchrotron and neutron radiation, and last but not least, the proposition of new algorithms. Innovations were not instantly accepted this being true for all the whole powder pattern fitting methods including the Rietveld and the decomposition methods or could not be applied immediately to every radiation source or diffractometer the hardware before adaptations made by an essential category of crystallographers being conceivers and developers of the software. Ancestors of the WPPD methods extracted peak intensities without the cell restraint, so that each peak position was a parameter to be refined as well as the peak intensity, the peak shape and its width. This is still useful if the aim is the search for the peak positions for indexing, though derivative methods can make that peak-position-hunting job faster. Taking advantage of the indexing see a recent review paper by Bergmann et al. 2004, new WPPD methods, applying cell restraint to the peak position, opened the door to a long list of new possibilities and applications including first indexing confirmation which are detailed in this paper. However, only some selected application references will be provided because the number of papers involved is quite high and increasing more than 2000 texts specify the use of WPPD methods. Contributions from Rennes by Louer’s group from 1987 to 1993 will be especially enlightened, not forgetting the other players during that same time, restraining generally to the structure determinations by powder diffractometry SDPD applications published in the early stages of this retrospection because the subsequent activity increased too considerably, by more than 850 SDPDs in the last ten years. If only a partial review of applications can be given, the evolution of the methods will be discussed as completely as possible.