| 1 | <html> |
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| 2 | <head> |
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| 3 | <META NAME="Author" CONTENT="Brian H. Toby"> |
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| 4 | <title>EXPGUI Tutorial #6</title> |
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| 5 | </head> |
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| 6 | <BODY BGCOLOR="#FFFFFF"> |
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| 7 | |
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| 8 | <A HREF=http://www.ncnr.nist.gov> |
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| 17 | <A Href="http://www.ncnr.nist.gov/programs/crystallography/software/tclpkgs.html"> |
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| 20 | </CENTER></A> |
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| 21 | <hr> |
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| 22 | |
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| 23 | <CENTER> |
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| 24 | <H1>EXPGUI Tutorial: Garnet (part 6)</H1> |
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| 25 | </CENTER> |
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| 26 | <H1> |
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| 27 | 6. Additional Least-Squares Refinement |
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| 28 | </H1> |
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| 29 | The initial fit obtained in the previous runs is not particularly good. The |
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| 30 | <A HREF="garnet5.html#diff">"first derivative"</A> |
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| 31 | appearance in the difference plot peaks indicates that |
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| 32 | peaks are not correctly positioned, so refining the lattice parameters and |
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| 33 | zero point correction are the first order of business. |
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| 34 | <BR clear=left><P> |
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| 35 | <img src="fit1.gif"> |
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| 36 | <BR clear=left> |
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| 37 | While normally I would recommend refining the lattice first and then |
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| 38 | in a subsequent refinement cycle also refining the zero point correction |
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| 39 | in a later cycle of refinement, with a cubic cell and a small |
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| 40 | zero-point error it is possible to refine both together. Set the refinement |
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| 41 | flag for the lattice (shown above) and the zero point correction (shown below). |
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| 42 | <BR clear=left><P> |
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| 43 | <img src="fit2.gif"> |
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| 44 | <BR clear=left><P> |
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| 45 | and then start GENLES, as was done before. |
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| 46 | <BR clear=left><P> |
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| 47 | <img src="fit3.gif"> |
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| 48 | <BR clear=left> |
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| 49 | A dramatic improvement is seen both through the agreement factors (above) |
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| 50 | and as seen in the plot (below). |
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| 51 | <BR clear=left><P> |
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| 52 | <a name="ticks"><img src="fit4.gif"></a> |
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| 53 | <BR clear=left><P> |
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| 54 | At this point it is a good idea to run POWPREF and GENLES again, |
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| 55 | since peak positions have moved. This causes the Reduced Chi**2 to drop from |
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| 56 | 14.9 to 13.5. |
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| 57 | |
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| 58 | <P><BR>(.EXP Snapshot: |
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| 59 | <A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/garnet6.exp"> |
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| 60 | garnet6.exp |
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| 61 | </A>) |
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| 62 | |
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| 63 | <BR clear=left><HR> |
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| 64 | Previous step: <A HREF="garnet5.html"> |
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| 65 | Plot data using POWPLOT or LIVEPLOT</A> |
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| 66 | </A> |
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| 67 | <BR> |
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| 68 | Next step: <A HREF="garnet7.html"> |
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| 69 | Define Constraints</A> |
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| 70 | |
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| 71 | <BR clear=left> |
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| 72 | <HR><p><a href="http://www.ncnr.nist.gov/staff/toby/">Brian Toby</a> (<a href="mailto:brian.toby@nist.gov">Brian.Toby@NIST.GOV</a>) |
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| 73 | <BR clear=left> |
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| 74 | $Revision: 173 $ $Date: 2009-12-04 23:01:37 +0000 (Fri, 04 Dec 2009) $ |
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| 75 | </body> |
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| 76 | </html> |
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