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
This exercise provides a tutorial example of how to use the
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
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 Macintosh (older unsupported versions exist for Silicon Graphics/IRIX computers).
The example figures show in this tutorial were generated in Unix,
but virtually all GSAS and EXPGUI operation is identical between Unix, Mac 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.
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
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
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.
Create a GSAS Experiment File
Adding a phase
Specifying Powder Diffraction Data (Adding a Histogram)
Changing the Background Function
Initial Fitting: Refine Scale Factor and Background
Plotting the Initial Fit
Fitting the Unit Cell
Fitting the Diffractometer Zero Correction
Initial Fitting of Profile Parameters
Group Uiso parameters &
Refine coordinates and Overall Uiso
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
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
EXPGUI is written by Brian H. Toby of the NIST Center for Neutron Research,
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.
$Revision: 1.3 $ $Date: 2003/07/18 18:32:08 $