Fig 1: Screen-shot of PDFgui during a refinement of multiple data-sets
C. L. Farrow, P. Juhas, J. W. Liu, D. Bryndin, E. S. Bozin, J. Bloch, and S. J. L. Billinge (Michigan State University)
To understand and control the properties of materials it is necessary to have a detailed knowledge of their atomic structure. Increasingly, we are interested in exploiting materials with complex structures on the nanoscale, which presents special experimental challenges as crystallography, the standard approach to structure determination, loses its power on these length-scales. Alternative methods sensitive to nanoscale order are under development. The diffraction sub-group of the DANSE project recently released a new software tool that will have a large impact in nanostructure characterization using x-ray and neutron diffraction.
Fig 1: Screen-shot of PDFgui during a refinement of multiple data-sets
The diffraction data are Fourier transformed to obtain the atomic pair distribution function (PDF) and analyzed by quantitatively fitting nanostructure models to the data. An example of the tool, PDFgui [1], is shown in the accompanying flash movie. It is being used to study the evolution of local structural parameters through a structural phase transition. Neutron-derived PDFs have been collected over a temperature range from room temperature to 880 K for the strongly correlated electron material, LaMnO3. PDFgui allows rapid setup of individual and multiple refinements. The first refinement is initialized using the intuitive gui (graphical user interface) tools, and the structure visualized using the built-in structure visualizing tool. At this point the temperature-series macro is chosen allowing multiple (in this case, 10) structure refinements to be selected and refined automatically. The flexible built-in plotting capabilities allows a refined parameter to be plotted during this process as shown. The structural phase transition is clearly visible as a cusp in this parameter, which happens to be a thermal factor on oxygen ions in the structure. This program will be implemented at the POWGEN3 and NOMAD diffractometers at SNS with macros to allow real-time parametric refinements similar to those shown, allowing researchers to make scientific discoveries during data collection, at the beam-time, and not a-posteriori after the beamtime is over. PDFgui has numerous extensions, beyond those shown in the movie that are described in more detail in Ref. [1]. The beta version of PDFgui is now in public release, available for Windows, Linux and Mac OSX platforms, and in the first month has been downloaded 220 times. More information can be found at http://www.diffpy.org.