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1<html>
2<head>
3   <title>EXPGUI Alumina Tutorial, part #7</title>
4</head>
5
6<style>
7A:link {text-decoration:none}
8A:vlink {text-decoration:none}
9</style>
10
11<BODY BGCOLOR="#FFFFFF"
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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<TR><TH><A  Href="index.html">Intro</A> 
23</TH><TH><A Href="alumina8.html">Next page</A>
24</TH><TH><A Href="alumina6.html">Previous page</A>
25</TH></TR></TABLE><BR CLEAR=ALL>
26
27
28<CENTER>
29<H1><HR noshade width="75%" size="2" align="center">
30GSAS/EXPGUI Alumina tutorial (part 7)<BR>
31Fitting the Unit Cell
32<HR noshade width="75%" size="2" align="center">
33</H1>
34</CENTER>
35In the previous tutorial, we saw that the unit cell was not
36doing a very good job of indexing the diffraction peaks.
37Since a large number of peaks did have computed reflections
38at least partly overlapping the observed peaks, we are close enough
39to fit the unit cell parameters using a refinement. If that were not true --
40the tick marks did not fall in the range of the peaks, refinement
41would not be possible.
42<P><HR noshade width="50%" size="4" align="center"><P>
43
44One the Phase panel, turn on the flag to refine the unit cell parameters
45(upper right), as shown below.
46<P>
47<img alt="tutorial screen dump" src="7a.gif" align=RIGHT>
48<BR clear=all>
49<P><HR noshade width="50%" size="4" align="center"><P>
50Also, increase the number of cycles to 6.
51<P>
52<img alt="tutorial screen dump" src="7b.gif" align=RIGHT>
53<BR clear=all>
54<P><HR noshade width="50%" size="4" align="center"><P>
55
56Run GENLES (as before). Note there are now 9 parameters
57(scale, 6 background, + 2 cell) and that the fit significantly improves, as
58seen below.
59<P>
60<img alt="tutorial screen dump" src="7c.gif" align=RIGHT>
61<BR clear=all>
62(As before, press Enter & load the modified experiment file into EXPGUI.)
63
64<P><HR noshade width="50%" size="4" align="center"><P>
65
66The peak positions are fit much better than before, as seen in the LIVEPLOT
67output below.
68<P><img alt="tutorial screen dump" src="7d.gif" align=RIGHT>
69<BR clear=all>
70<P><HR noshade width="50%" size="4" align="center"><P>
71
72Since the peak positions have changed significantly, we now need to rerun
73POWPREF and use the new peak positions to decide which data points need
74to be indexed with each reflection.
75<P>
76<img alt="tutorial screen dump" src="7e.gif" align=RIGHT>
77<BR clear=all>
78<P><HR noshade width="50%" size="4" align="center"><P>
79
80It is not strictly necessary to run GENLES again, but note that CHI-squared
81improves from 70 to 60 just from the better indexing.
82<P>
83<img alt="tutorial screen dump" src="7f.gif" align=RIGHT>
84<BR clear=all>
85<HR>
86Previous: <A HREF="alumina6.html">
87Plotting the Initial Fit
88</A>
89<BR>
90Next step: <A HREF="alumina8.html">
91Fit the Diffractometer Zero Correction
92</A>
93
94<HR>
95</blockquote>
96<P><font size=-1><A HREF="MAILTO:crystal@NIST.gov?subject=WWW page <?=$PHP_SELF?>">Comments, corrections or questions: crystal@NIST.gov</A></font><BR>
97<font size=-1><? lastmod(); ?> 
98<BR>$Revision: 930 $ $Date: 2009-12-04 23:14:35 +0000 (Fri, 04 Dec 2009) $</font>
99</body>
100</html>
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