Solving the Structures of Polycrystalline Materials: from the Debye-Scherrer Camera to SwissFEL

Abstract

The development of powder diffraction (PD) techniques for structure analysis is traced from its inception almost 100 years ago to the present day, with a brief glimpse of what SwissFEL can contribute in the near future. Although PD data were used in the early days to deduce some simple high-symmetry structures, it was not until computers, instrumentation and synchrotrons arrived on the scene that the true potential of PD data could be realized. In the last 25 years, PD has blossomed into a viable method, not only for structure refinement, but also for structure solution. This means that scientists with polycrystalline materials that cannot be grown as single crystals can still obtain the structural information they need. Historically, structure solution from PD data began with model building, progressed through the application of single-crystal methods to simpler structures and the adaptation of those methods to the special problems posed by PD data, the development of automated model-building algorithms, and most recently to the application of charge flipping. As X-ray sources and detectors continue to develop, the boundary between a powder and a single crystal is narrowing. Laue microdiffraction techniques and the prospects offered by SwissFEL will allow single-crystal data to be collected on some polycrystalline materials.

Structure determination from powder diffraction data: past, present and future challenges

“…A brief overview of the historical development of structure determination is given in order to follow the development of the ideas and approaches that led to the contemporary methods of structure determination using low resolution powder diffraction data. More detailed review of this subject has been published recently .…”

Complex Hydrides – When Powder Diffraction needs Help

“…Only in the rarest cases is a sample a well crystallized and single phase. [14] The different approaches to structure solution from powder samples are covered in another contribution to this issue [15] and will not be elaborated on here. Most commonly the multi-phase nature and poor crystallinity of the investigated powders lead to significant peak broadening and overlap and prevent structural solution in reciprocal space.…”

“…Therefore, a chemically disordered site tends to change composition as a function of temperature. We inspected diffraction data at intervals of 1.5 K but the stoichiometry of Li 3 MZn 5 (BH 4 ) 15 does not vary at all between 150 K and the decomposition of the compound at 393 K, nor does the chemical composition of 6g. This suggested that only an average view of the chemically disordered Li-Mg occupancy on 6g is seen by diffraction.…”