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| 3 | <title>EXPGUI Alumina Tutorial, part #9</title> |
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| 16 | include("/var/www/include/navigation.inc"); |
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| 17 | include("/var/www/include/utility.inc"); |
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| 19 | <blockquote> |
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| 20 | |
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| 21 | <TABLE BORDER BGCOLOR="#FFFF40" ALIGN=RIGHT> |
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| 22 | <TR><TH><A Href="index.html">Intro</A> |
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| 23 | </TH><TH><A Href="alumina10.html">Next page</A> |
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| 24 | </TH><TH><A Href="alumina8.html">Previous page</A> |
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| 25 | </TH></TR></TABLE><BR CLEAR=ALL> |
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| 26 | |
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| 27 | |
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| 28 | <CENTER> |
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| 29 | <H1><HR noshade width="75%" size="2" align="center"> |
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| 30 | GSAS/EXPGUI Alumina tutorial (part 9)<BR> |
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| 31 | Initial Fitting of Profile Parameters |
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| 32 | |
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| 33 | <HR noshade width="75%" size="2" align="center"> |
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| 34 | </H1> |
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| 35 | </CENTER> |
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| 36 | After the unit cell is fit, the next step is typically |
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| 37 | to improve the crystallographic |
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| 38 | model if the profile is well-fit. In this case, however, |
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| 39 | we cannot do that since |
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| 40 | the high-angle peaks are much broader than the calculated pattern. |
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| 41 | This is demonstrated clearly in the LIVEPLOT output shown below. |
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| 42 | <P> |
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| 43 | <img alt="tutorial screen dump" src="9a.gif" align=RIGHT> |
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| 44 | <BR clear=all> |
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| 45 | <blockquote><I> |
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| 46 | The plot above shows several problems that need to be addressed by |
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| 47 | refining parameters. As noted before, the observed peaks are |
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| 48 | significantly broader than the calculated, this is addressed by |
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| 49 | optimizing the peak shape parameters. Also, the relative intensities |
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| 50 | in the calculated pattern does not match the observed intensities, |
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| 51 | this can potentially be addressed by refinement of coordinates and, |
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| 52 | to a lesser extent, individual displacement (temperature) parameters. |
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| 53 | It is also worth noting that the computed background is too high at |
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| 54 | higher two-theta values. This may be improved by refinement of the |
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| 55 | background when the peak intensities are better fit or may require the |
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| 56 | addition of more background parameters. In any case, neither the coordinates |
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| 57 | or the background can be effectively optimized until a reasonable fit is |
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| 58 | obtained for the peak shape. |
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| 59 | </I></blockquote> |
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| 60 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 61 | |
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| 62 | In the Profile panel shown below, |
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| 63 | the three Gaussian peak width terms (GU, GV & GW, also |
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| 64 | known as the Cagilloti terms U, V & W), are added to the refinement. |
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| 65 | <P><img alt="tutorial screen dump" src="9b.gif" align=RIGHT> |
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| 66 | <BR clear=all> |
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| 67 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 68 | |
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| 69 | After the flags are set for the profile terms, the GENLES program is |
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| 70 | rerun. At this point, the fit improves considerably. |
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| 71 | |
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| 72 | <P><img alt="tutorial screen dump" src="9c.gif" align=RIGHT> |
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| 73 | <BR clear=all> |
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| 74 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 75 | |
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| 76 | If the fit is examined closely at this stage with LIVEPLOT (as shown below), |
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| 77 | it can be seen that the peaks at high angle appear to be trucated on each side. |
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| 78 | This is due to the fact that the peak profiles have gotten much broader, |
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| 79 | but POWPREF has not yet been rerun, so not enough data points are included |
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| 80 | in the computation. |
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| 81 | <P><img alt="tutorial screen dump" src="9d.gif" align=RIGHT> |
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| 82 | <BR clear=all> |
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| 83 | <blockquote><I> |
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| 84 | This sort of problem occurs fairly commonly for neophyte GSAS users. The |
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| 85 | way to avoid this is to remember to rerun POWPREF after any significant |
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| 86 | change in the lattice parameters, zero correction or peak profile terms. |
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| 87 | Also be sure to run POWPREF after adding phases, histograms or changing |
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| 88 | the excluded regions. |
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| 89 | </I></blockquote> |
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| 90 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 91 | |
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| 92 | Run POWPREF and then GENLES again. Note that still more improvement is |
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| 93 | seen in the Chi-squared and R-factors. |
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| 94 | <P><img alt="tutorial screen dump" src="9f.gif" align=RIGHT> |
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| 95 | <BR clear=all> |
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| 96 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 97 | |
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| 98 | Now LIVEPLOT shows a normally-shaped diffraction peak. |
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| 99 | <img alt="tutorial screen dump" src="9g.gif" align=RIGHT> |
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| 100 | <BR clear=all> |
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| 101 | <HR> |
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| 102 | Previous: <A HREF="alumina8.html"> |
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| 103 | Fit the Diffractometer Zero Correction |
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| 104 | </A> |
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| 105 | <BR> |
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| 106 | Next step: <A HREF="alumina10.html"> |
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| 107 | Group U<sub>iso</sub> parameters; Refine coordinates and Overall U<sub>iso</sub>. |
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| 108 | </A> |
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| 109 | |
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| 110 | <HR> |
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| 111 | </blockquote> |
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| 112 | <P><font size=-1><A HREF="MAILTO:crystal@NIST.gov?subject=WWW page <?=$PHP_SELF?>">Comments, corrections or questions: crystal@NIST.gov</A></font><BR> |
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| 113 | <font size=-1><? lastmod(); ?> |
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| 114 | <BR>$Revision: 930 $ $Date: 2009-12-04 23:14:35 +0000 (Fri, 04 Dec 2009) $</font> |
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| 115 | </body> |
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| 116 | </html> |
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