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GSAS/EXPGUI Alumina example (Intro)

What's this all about?

The goal of Rietveld analysis is to fit a structural model ("crystal structure") to powder diffraction data. To do this requires determining the structural parameters [unit cell, atom positions and displacement (thermal) parameters, etc.] for all crystalline phases present, as well as a variety of instrumental and sample parameters that describe the experimental and sample conditions: scale factors, peak broadening, the background, preferred orientation, etc. In most cases Rietveld analysis is performed to determine the structural parameters, but increasingly, the method is also used to determine relative amounts of the crystallographic phases, the amount and type of peak broadening, the preferred orientation, or similar types of sample characterization.

This exercise provides a tutorial example of how to use the GSAS software package with the EXPGUI interface to perform Rietveld analysis. The material chosen for this exercise, corundum (aka alumina, sapphire or ruby), has a simple structure, so the starting coordinates have been distorted so that there is an improvement obtained by performing the fit. Likewise, this sample exhibited virtually no sample-related broadening, so the instrumental peak profile was altered so that the peak profile terms would not agree with the data, again so that the exercise would demonstrate the sorts of steps needed for typical Rietveld refinements. The tutorial consists of 11 web pages, each has a series of related steps. There are also editorial comments, that explain a point further or explain how these steps might be applied differently in another case. These comments are in specified in italic type.

Rietveld analysis works using non-linear least-squares fitting to optimize (refine) parameters. This means that we must start with approximate values for all parameters that will be fit. We then allow the software to optimize a small subset of the parameters -- a minimal number of parameters that must be fit before any progress can be made. Slowly, additional parameters are selected to be refined, until all parameters in the model (if the data support that) are refined. Despite the simplicity of the material, this exercise demonstrates many of the procedures needed for more complex materials.

EXPGUI and GSAS run on Windows, Linux and IRIX (Silicon Graphics computers). The example figures show in this tutorial were generated in Unix, but virtually all GSAS and EXPGUI operation is identical between Unix and Windows. Note also that the appearance of EXPGUI changes slightly as new features are added. The screen images do not exactly match the current version of EXPGUI.

Getting Started

To perform this tutorial on your own computer, you will need to have GSAS and EXPGUI loaded on your computer (see installation links on the EXPGUI home page). You will also need three files that are referenced in the following web pages: These three files can be downloaded as a single .ZIP file, or accessed via anonymous ftp from site ftp.ncnr.nist.gov in directory /pub/cryst/gsas/tutorials

Note: While it is possible to have your working GSAS files (i.e. the .EXP file, etc.) in a separate directory from the raw data file(s), I discourage this practice, as it then becomes quite difficult to later copy or move the .EXP file from one directory or computer to another. For this reason, I suggest copying these files into the directory where you will keep your GSAS files.

Tutorial Outline

  1. Create a GSAS Experiment File
  2. Adding a phase
  3. Specifying Powder Diffraction Data (Adding a Histogram)
  4. Changing the Background Function
  5. Initial Fitting: Refine Scale Factor and Background
  6. Plotting the Initial Fit
  7. Fitting the Unit Cell
  8. Fitting the Diffractometer Zero Correction
  9. Initial Fitting of Profile Parameters
  10. Group Uiso parameters & Refine coordinates and Overall Uiso
  11. Finishing Up

Sample files

Acknowledgments

GSAS is written by Allen C. Larson and Robert B. Von Dreele, MS-H805, Los Alamos National Laboratory, Los Alamos, NM 87545. Problems, questions or kudos concerning GSAS should be sent to Robert B. Von Dreele at vondreele@anl.gov

GSAS is Copyright, 1984-2003, The Regents of the University of California. The GSAS software was produced under a U.S. Government contract (W-7405-ENG-36) by the Los Alamos National Laboratory, which is operated by the University of California for the U.S. Department of Energy. The U.S. Government is licensed to use, reproduce, and distribute this software. Permission is granted to the public to copy and use this software without charge, provided that this notice and any statement of authorship are reproduced on all copies. Neither the Government nor the University makes any warranty, express or implied, or assumes any liability or responsibility for the use of this software.

EXPGUI is written by Brian H. Toby of the NIST Center for Neutron Research, Brian.Toby@NIST.GOV

EXPGUI is not subject to copyright. Have fun with it.

Neither the U.S. Government nor any author makes any warranty, expressed or implied, or assumes any liability or responsibility for the use of this information or the software described here. Brand names cited here are used for identification purposes and do not constitute an endorsement by NIST.

Comments, corrections or questions: crystal@NIST.gov
Last modified 18-July-2003
$Revision: 1.3 $ $Date: 2003/07/18 18:32:08 $