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1<html>
2<head>
3   <title>EXPGUI Alumina Tutorial, part #9</title>
4</head>
5
6<style>
7A:link {text-decoration:none}
8A:vlink {text-decoration:none}
9</style>
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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="alumina10.html">Next page</A>
24</TH><TH><A Href="alumina8.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 9)<BR>
31Initial Fitting of Profile Parameters
32
33<HR noshade width="75%" size="2" align="center">
34</H1>
35</CENTER>
36After the unit cell is fit, the next step is typically
37to improve the crystallographic
38model if the profile is well-fit. In this case, however,
39we cannot do that since
40the high-angle peaks are much broader than the calculated pattern.
41This is demonstrated clearly in the LIVEPLOT output shown below.
42<P>
43<img alt="tutorial screen dump" src="9a.gif" align=RIGHT>
44<BR clear=all>
45<blockquote><I>
46The plot above shows several problems that need to be addressed by
47refining parameters. As noted before, the observed peaks are
48significantly broader than the calculated, this is addressed by
49optimizing the peak shape parameters. Also, the relative intensities
50in the calculated pattern does not match the observed intensities,
51this can potentially be addressed by refinement of coordinates and,
52to a lesser extent, individual displacement (temperature) parameters.
53It is also worth noting that the computed background is too high at
54higher two-theta values. This may be improved by refinement of the
55background when the peak intensities are better fit or may require the
56addition of more background parameters. In any case, neither the coordinates
57or the background can be effectively optimized until a reasonable fit is
58obtained for the peak shape.
59</I></blockquote>
60<P><HR noshade width="50%" size="4" align="center"><P>
61
62In the Profile panel shown below,
63the three Gaussian peak width terms (GU, GV & GW, also
64known as the Cagilloti terms U, V & W), are added to the refinement.
65<P><img alt="tutorial screen dump" src="9b.gif" align=RIGHT>
66<BR clear=all>
67<P><HR noshade width="50%" size="4" align="center"><P>
68
69After the flags are set for the profile terms, the GENLES program is
70rerun. At this point, the fit improves considerably.
71
72<P><img alt="tutorial screen dump" src="9c.gif" align=RIGHT>
73<BR clear=all>
74<P><HR noshade width="50%" size="4" align="center"><P>
75
76If the fit is examined closely at this stage with LIVEPLOT (as shown below),
77it can be seen that the peaks at high angle appear to be trucated on each side.
78This is due to the fact that the peak profiles have gotten much broader,
79but POWPREF has not yet been rerun, so not enough data points are included
80in the computation.
81<P><img alt="tutorial screen dump" src="9d.gif" align=RIGHT>
82<BR clear=all>
83<blockquote><I>
84This sort of problem occurs fairly commonly for neophyte GSAS users. The
85way to avoid this is to remember to rerun POWPREF after any significant
86change in the lattice parameters, zero correction or peak profile terms.
87Also be sure to run POWPREF after adding phases, histograms or changing
88the excluded regions.
89</I></blockquote>
90<P><HR noshade width="50%" size="4" align="center"><P>
91
92Run POWPREF and then GENLES again. Note that still more improvement is
93seen in the Chi-squared and R-factors.
94<P><img alt="tutorial screen dump" src="9f.gif" align=RIGHT>
95<BR clear=all>
96<P><HR noshade width="50%" size="4" align="center"><P>
97
98Now LIVEPLOT shows a normally-shaped diffraction peak.
99<img alt="tutorial screen dump" src="9g.gif" align=RIGHT>
100<BR clear=all>
101<HR>
102Previous: <A HREF="alumina8.html">
103Fit the Diffractometer Zero Correction
104</A>
105<BR>
106Next step: <A HREF="alumina10.html">
107Group U<sub>iso</sub> parameters; Refine coordinates and Overall U<sub>iso</sub>.
108</A>
109
110<HR>
111</blockquote>
112<P><font size=-1><A HREF="MAILTO:crystal@NIST.gov?subject=WWW page <?=$PHP_SELF?>">Comments, corrections or questions: crystal@NIST.gov</A></font><BR>
113<font size=-1><? lastmod(); ?> 
114<BR>$Revision: 930 $ $Date: 2009-12-04 23:14:35 +0000 (Fri, 04 Dec 2009) $</font>
115</body>
116</html>
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