| 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 #7</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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| 16 | <CENTER> |
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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 7)</H1> |
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| 25 | </CENTER> |
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| 26 | <H1> |
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| 27 | 7. Define constraints |
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| 28 | </H1> |
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| 29 | The next step in the refinement is to vary atomic positions, displacement |
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| 30 | (thermal amplitude) parameters and since two sites are |
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| 31 | shared in this material, site occupancies. It is almost never possible to |
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| 32 | refine different displacement parameters or position parameters for |
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| 33 | two atoms that share a site, so one must use constraints to require that |
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| 34 | coordinates and thermal parameters be grouped. With only a single data set |
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| 35 | it is only possible to refine a single occupancy value for a site, so it is |
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| 36 | also necessary to constrain the site occupancies to they add to 1. |
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| 37 | <P> |
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| 38 | For this example, we need to constrain the pairs of atoms that share a site, |
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| 39 | Fe2/Al3 and Al4/Fe5. |
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| 40 | This can be done using the constraints pane. In this case, both atoms are |
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| 41 | on special positions so the coordinates are fixed by symmetry and do |
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| 42 | not need to be constrained. This, however, |
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| 43 | will be ignored to illustrate how coordinates are constrained. |
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| 44 | <BR clear=left> |
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| 45 | <img src="constr1.gif"> |
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| 46 | <BR clear=left> |
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| 47 | <img src="constr2.gif" align=left> |
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| 48 | <img src="constr3.gif" align=right> |
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| 49 | Pressing the "New Constraint" button causes the box to the left to appear. |
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| 50 | Select phase 1 at the top, the atoms Fe2 and Al3 by "dragging" |
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| 51 | over them with the mouse (clicking with the mouse while holding the |
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| 52 | control key also works). Select the "XYZU" variable to group x, y, z and Uiso |
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| 53 | for the selected atoms, as shown to the right. Press "Save" and the four |
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| 54 | constraints are generated and appear on the Constraints pane. |
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| 55 | <BR clear=left> |
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| 56 | <BR clear=right> |
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| 57 | <img src="constr4.gif" align=left> |
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| 58 | To constrain the fractional occupancies, again press the "New Constraint" |
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| 59 | button and then press the "New Column" button, to create a second |
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| 60 | constraint box. Select phase 1 for both boxes, Fe2 in one box and Al3 |
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| 61 | in the other variable "Frac" in the left-most box. Use |
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| 62 | any values for the multiplier that are equal in magnitude |
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| 63 | but opposite in sign. |
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| 64 | Press "Save" and this constraint is also generated and it appears on the |
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| 65 | Constraints pane, as is shown below. |
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| 66 | <BR clear=left> |
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| 67 | <img src="constr5.gif"> |
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| 68 | <BR clear=left> |
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| 69 | The previous two steps are then repeated for Al4 and Fe5, to generate an |
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| 70 | addtional five more constraints, as is shown below. |
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| 71 | <BR clear=left> |
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| 72 | <img src="constr6.gif"> |
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| 73 | <BR clear=left> |
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| 74 | |
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| 75 | <P><BR>(.EXP Snapshot: |
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| 76 | <A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/garnet7.exp"> |
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| 77 | garnet7.exp |
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| 78 | </A>) |
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| 79 | |
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| 80 | <BR clear=left><HR> |
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| 81 | Previous step: <A HREF="garnet6.html"> |
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| 82 | Additional Least-Squares Refinement</A> |
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| 83 | <BR> |
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| 84 | Next step: <A HREF="garnet8.html"> |
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| 85 | Vary atomic parameters</A> |
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| 86 | </A> |
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| 87 | |
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| 88 | |
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| 89 | <BR clear=left> |
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| 90 | <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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| 91 | <BR clear=left> |
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| 92 | $Revision: 174 $ $Date: 2009-12-04 23:01:38 +0000 (Fri, 04 Dec 2009) $ |
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| 93 | </body> |
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| 94 | </html> |
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