The following is TOF NIST Silicon 640b. (Cell = 5.430940 +/- 0.000035) collected on ISIS HRPD at the Rutherford Laboratory. It is assumed that you have the following information:
Click here to obtain the ISIS HRPD NIST Silicon 640b data and Rietica input files
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Insert the Crystal structure information for your "position standard" (NIST Silicon)
via the Model, Phases menu.
In this case we will be initially be doing a Le Bail fit, so no atom position information is required.
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The Incident Spectrum information is
inserted into Rietica via the Model, Histograms menu
then selecting the Incident Spectrum button.
With this data having been corrected for the incident spectrum, only a0=1 has to be inputted.
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The starting Zero, Difa and Difc constants are inserted into Rietica
via the Model, Histograms menu. Starting values could be obtained
from a previous calibration done on the neutron station, or from the
station manager.
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The alpha0, alpha1, beta0, beta1, Sig1 and Gam1 are inserted via the
Model, Sample menu. Again, try reasonable values based on
previous refinements or suggestions from the station manager.
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Now, with only the background parmaters, perform an initial set of cycles to get the Le Bail intensities to fit. You may have to tweak the Zero, Difa and Difc constants if they are too far off. In the following starting file, they are obviously too far away from their true values to be able to lock in.
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If the HKLs are too far from the approximately true values, have a play (or try finding some better previously used values) to get the HKLs to roughly match the peaks. The following is closer but there is little likelihood of stable refinement unless the HKLs are hitting the tails of the peaks. They are just missing them here.
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After further trial and error changing of Difc and Difa values, we are now ready to start the refinement.
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Now from the Model, Histograms menu, refine Difc after which the HKLs should hopefully be reasonably well aligned with the peaks.
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From the Model, Histograms menu, refine Difa and Zero after which the HKLs should hopefully now match the HKLs to the Silicon peaks over the whole TOF range.
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As this is ISIS HRPD data, via the Model Sample only refine the alpha1, beta0 and beta1 Peakshape parameters (not alpha0). After which the profile fit should drastically improve.
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Via the Model Sample menu, refine the Sig1 Peakshape parameter - and add in the Gam1 parameter. The Le Bail fitting should now be pretty much complete and give an almost flat difference line. Now, you may want to add in the Silicon structural co-ordinates, and then complete the structure refinement. The following information from Brett Hunter tells you how to fit other TOF patterns using the resulting fit to the Silicon standard.
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From Brett Hunter: 1) DIFC, DIFA, ZERO are never refined during a normal course of refinement - only when calibrating (no lattice parameter refinement in that case). They correlate too strongly (1-to-1) to the lattice parameters, and should, for TOF, be true instrument parameters. 2) the alpha's and beta's should also be instrumental, unless you believe that you have asymmetry, such as a compositional variation in your sample. Usually only alpha-1, beta-0 and beta-1 are non-zero in any case. For HRPD-ISIS the alpha's are larger than say SEPD-IPNS due to the sharp rise and high resolution. You may in the case of the alpha-1 get a large variation (with little or no improvement in fit) for HRPD-ISIS. As it doesn't affect anyhting - don't worry. 3) you may near the end of a refinement turn them on alpha/beta's to see what happens - sometime a small improvement in fit is obtained. Unless you believe there is a physical reason for it, don't change them from the instrumental values. 4) if you are far from the ideal peakshape/fit, a suggestion is to keep sig-1 fixed at a readonable value, say 10-15. Fix up as many of the other problems (such as lattice parameters etc), then refine sig-1. After that try gam-1. Sometimes it is also best to put it further off a zero value, say at a value of 10, and then refine, that to refine directly from 0.0. After that you can try sig-2 and gam-2. 5) If your instrumental fit has no sig-0 and gam-0 components (eg HRPD-ISIS), then don't bother refining them until the end, if at all.
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