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# on 2002/07/18 20:45:10, toby did:
NIST alumina tutorial

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1<html>
2<head>
3   <title>GSAS/EXPGUI Alumina Tutorial Intro</title>
4</head>
5
6<style>
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14
15<?
16   include("/var/www/include/navigation.inc");
17   include("/var/www/include/utility.inc");
18?>
19<blockquote>
20
21<TABLE BORDER BGCOLOR="#FFFF40" ALIGN=RIGHT>
22</TH><TH><A Href="alumina1.html">First page in tutorial</A>
23</TH></TR></TABLE><BR CLEAR=ALL>
24
25<CENTER>
26<H1><HR noshade width="75%" size="2" align="center">
27GSAS/EXPGUI Alumina example (Intro)
28<HR noshade width="75%" size="2" align="center">
29</H1>
30</CENTER>
31<H1>What's this all about?</H1>
32The goal of Rietveld analysis is to fit a structural model
33("crystal structure") to powder diffraction data. To do this requires
34determining the structural parameters [unit cell,
35atom positions and displacement (thermal) parameters, etc.]
36for all crystalline phases present, as well as a variety of
37instrumental and sample parameters that describe the experimental and
38sample conditions: scale factors, peak broadening, the background,
39preferred orientation, etc.
40In most cases Rietveld analysis is performed to determine the structural
41parameters, but increasingly, the method is also used to determine relative
42amounts of the crystallographic phases, the amount and type of peak
43broadening, the preferred orientation, or similar types of sample
44characterization.
45<P>
46This exercise provides a tutorial example of how to use the
47<A href="http://www.ncnr.nist.gov/xtal/software/gsas.html">GSAS</a>
48software package
49with the
50<A href="http://www.ncnr.nist.gov/xtal/software/expgui/expgui_intro.html">EXPGUI</A>
51interface to perform Rietveld analysis. The material chosen for this exercise,
52corundum (aka alumina,
53sapphire or ruby),
54has a simple structure, so the starting coordinates have been distorted
55so that there is an improvement obtained by performing the fit.
56Likewise, this sample exhibited virtually no sample-related broadening,
57so the instrumental peak profile was altered so that the peak profile
58terms would not agree with the data, again so that the exercise would
59demonstrate the sorts of steps needed for typical Rietveld refinements.
60The tutorial consists of 11 web pages, each has a series of related
61steps. There are also editorial comments, that explain a point further
62or explain how these steps might be applied differently in another case.
63<I>These comments are in specified in italic type</I>.
64<P>
65Rietveld analysis works using non-linear least-squares fitting to
66optimize (refine) parameters. This
67means that we must start with approximate values for all parameters that
68will be fit. We then allow the software to optimize a small subset of the
69parameters -- a minimal number of parameters that must be fit before any
70progress can be made. Slowly, additional parameters are selected to be
71refined, until
72all parameters in the model (if the data support that) are refined.
73Despite the simplicity of the material, this exercise
74demonstrates many of the procedures needed for more complex materials.
75<P>
76EXPGUI and GSAS run on Windows, Linux and IRIX (Silicon Graphics computers).
77The example figures show in this tutorial were generated in Unix,
78but virtually all GSAS and EXPGUI operation is identical between Unix and
79Windows.
80<P>
81
82
83<H1>Getting Started</H1>
84
85To perform this tutorial on your own computer, you will need to
86have GSAS and EXPGUI loaded on your computer (see
87<a href="http://www.ncnr.nist.gov/xtal/software/expgui/expgui.html">installation links on the EXPGUI home page</a>).
88<a name="downloads"></a>
89You will also need three files that are referenced in the following
90web pages:
91<UL>
92<LI>The Raw data:
93<A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/al2o3001.gsa">
94al2o3001.gsa</A>
95<LI>The instrument parameter file:
96<A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/bt1demo.ins">
97bt1demo.ins</A>
98<LI>A CIF file with unit cell and atomic parameters:
99<A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/alumina.cif">
100alumina.cif</A>
101
102
103<P>
104<blockquote>
105<B>Note:</B><I> 
106While it is possible to have your working GSAS files
107(for example the .EXP file) in a separate directory from
108the raw data file(s), I discourage this practice as it then
109becomes quite difficult to later copy or move the .EXP
110file from one directory or computer to another.
111For this reason, I suggest copying these files
112into the directory where you will create the Experiment
113(.EXP) file.
114</I><P>
115</blockquote>
116</blockquote>
117
118<H2>Tutorial Outline</H2>
119<OL>
120<LI><A HREF="alumina1.html">
121Create a GSAS Experiment File
122</A>
123<LI><A HREF="alumina2.html">
124Adding a phase
125</A>
126<LI><A HREF="alumina3.html">
127Specifying Powder Diffraction Data (Adding a Histogram)
128</A>
129<LI><A HREF="alumina4.html">
130Changing the Background Function
131</A>
132<LI><A HREF="alumina5.html">
133Initial Fitting: Refine Scale Factor and Background
134</A>
135<LI><A HREF="alumina6.html">
136Plotting the Initial Fit
137</A>
138<LI><A HREF="alumina7.html">
139Fitting the Unit Cell
140</A>
141<LI><A HREF="alumina8.html">
142Fitting the Diffractometer Zero Correction
143</A>
144<LI><A HREF="alumina9.html">
145Initial Fitting of Profile Parameters
146</A>
147<LI><A HREF="alumina10.html">
148Group U<sub>iso</sub> parameters &
149Refine coordinates and Overall U<sub>iso</sub></A>
150<LI><A HREF="alumina11.html">
151Finishing Up</A>
152</OL>
153
154<P>
155<H2>Sample files</H2>
156<UL>
157<LI><A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/al2o3001.gsa">
158al2o3001.gsa</A>
159-- the alumina neutron diffraction data
160<LI><A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/bt1demo.ins">
161bt1demo.ins</A>
162-- the instrument parameter file
163<LI><A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/alumina.cif">
164alumina.cif</A>
165-- a CIF file with unit cell and atomic parameters:
166</UL>
167<H2>Acknowledgments</H2>
168<A Href="http://www.ncnr.nist.gov/programs/crystallography/software/gsas.html">
169GSAS</A>
170is written by Allen C. Larson and <A HREF="http://www.nist.gov/cgi-bin/exit_nist.cgi?url=http://lansce.lanl.gov/lujan/staff12/vondreele.htm">
171Robert B. Von Dreele</A>, MS-H805,
172Los Alamos National Laboratory, Los Alamos, NM 87545. Problems, questions
173or kudos concerning GSAS should be sent to Robert B. Von Dreele at
174<A HREF="MAILTO:vondreele@lanl.gov">vondreele@lanl.gov</A>
175<P>GSAS is Copyright, 1984-1997, The Regents of the University of California.
176The GSAS software was produced under a U.S. Government contract (W-7405-ENG-36)
177by the Los Alamos National Laboratory, which is operated by the University
178of California for the U.S. Department of Energy. The U.S. Government is
179licensed to use, reproduce, and distribute this software. Permission is
180granted to the public to copy and use this software without charge, provided
181that this notice and any statement of authorship are reproduced on all
182copies. Neither the Government nor the University makes any warranty, express
183or implied, or assumes any liability or responsibility for the use of this
184software.
185<P>EXPGUI is written by Brian H. Toby of the NIST Center for Neutron Research,
186<A HREF="MAILTO:Brian.Toby@NIST.GOV">Brian.Toby@NIST.GOV</A> 
187
188<P>EXPGUI is not subject to copyright. Have fun with it.
189
190<p>Neither the U.S. Government nor any author makes any warranty,
191expressed or implied, or assumes any liability or responsibility
192for the use of this information or the software described
193here. Brand names cited here are used for
194identification purposes and do not constitute an endorsement by NIST.
195
196<P>
197<HR>
198</blockquote>
199<P><font size=-1><A HREF="MAILTO:crystal@NIST.gov?subject=WWW page <?=$PHP_SELF?>">Comments, corrections or questions: crystal@NIST.gov</A></font><BR>
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201<BR>$Revision: 629 $ $Date: 2009-12-04 23:09:22 +0000 (Fri, 04 Dec 2009) $
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