Branton J. Campbell (branton_campbell@byu.edu, Department of Physics & Astronomy, Brigham Young University, USA)
John S. O. Evans (john.evans@durham.ac.uk, Department of Chemistry, University of Durham, UK)
DESCRIPTION
The availability of reliable and user-friendly software tools has now made symmetry-mode analysis accessible to the general crystallographic community. Derived from group theory, symmetry-modes are an especially simple description of the degrees of freedom (e.g. displacive, occupational, magnetic) that arise when a crystalline solid undergoes a transition from high to low symmetry. Participants will receive hands-on training in how to (1) generate and visually explore the symmetry modes responsible for a distortion, (2) decompose a known structure into symmetry-mode amplitudes, and (3) directly refine a symmetry-mode model against diffraction data in TOPAS. The examples presented will involve complex oxides, though the methods presented can be extended to molecular and macromolecular materials. The training will be geared towards beginners.
PREPARING FOR THE WORKSHOP
Download and unzip this collection of structure and data files (smaintroduction.zip) to a convenient location on your computer, which will be used for hands-on tutorials. It is expected that each participant bring a laptop computer that is capable of running the following software. Some of the tutorial exercises are described on John Evans' Topas Tutorial website (see tutorials #36-#38) at http://www.dur.ac.uk/john.evans/topas_workshop/pcg_workshop_menu.htm.
ISODISTORT (http://stokes.byu.edu/isodistort.html): This online server-based software tool is intended to work in any web browser on any software platform. However, we are not always able to test less common browser/platform combinations. As you will perform all workshop exercises on your own laptop, please test ISODISTORT on your system before coming to the workshop. To test ISODISTORT, go to the home page (follow link above) and choose "Get started quickly with a distorted-perovskite example", which displays a well-known SrTiO3 distortion. Leaving all default parameters in place, simply click the "OK" button near the top of the page to open an interactive Java applet. The first time you open the applet, you will be asked if you trust the application to access your computer -- you only need to answer "yes" if you want to save images to disk. Use a mouse to rotate the structure and to explore the structural degrees of freedom available. If you encounter problems, please ensure that you have an up-to-date version of the web browser of your choice and that you have a reasonably-new (1.5 or later) version of Sun Java (http://java.com/en/download/index.jsp) correctly installed on your computer. Java is required in order to use the visualization tools. Note that many inexplicable problems can be resolved by simply restarting the browser.
TOPAS with jEdit: If you don't already have TOPAS installed on your computer, contact Arnt Kern (Arnt.Kern@bruker-axs.de) at Bruker AXS or Alan Coehlo (AlanCoelho@bigpond.com). You will also need to install the jEdit text editor and a collection of jEdit macros prepared by the University of Durham for conveniently building TOPAS input files. To do so, follow the instructions at http://www.dur.ac.uk/john.evans/topas_academic/jedit_setup.htm.