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Tutorials and Examples

LHPM-Rietica Rietveld for Win95/NT by Brett Hunter

Refining the instrumental components of Neutron Time of Flight data using Rietica-LHPM

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[Tutorials page] | [LHPM-Rietica Tutorial Index]

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:

  • TOF incident spectrum information or data that already has these corrections. (a0 to a8) (If your calculated vs observed intensities are all out of whack, it is most likely that you have not inserted the incident spectrum terms into Rietica)

    If your data has been corrected for incident spectrum, a0 will normally have the value of "1" with all other incident spectrum values set to 0.

    (Normally the Station manager would give you this information)

  • The UNITCELL of the standard (With this NIST Silicon 640b, the Cell = 5.430940 +/- 0.000035)

Click here to obtain the ISIS HRPD NIST Silicon 640b data and Rietica input files

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.

Inputting the crystal structure information

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.

Inputting the Incident Spectrum values

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.

Model, Histograms menu

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.

Model, Sample menu

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.

Initial cycles of the Le Bail fitting

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.

HKLs are better matching

After further trial and error changing of Difc and Difa values, we are now ready to start the refinement.

HKLs are better matching

Now from the Model, Histograms menu, refine Difc after which the HKLs should hopefully be reasonably well aligned with the peaks.

Refining Difc

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.

Refining Difa and Zero

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.

Refining alpha and beta components

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.

Le Bail fitted HRPD data

Le Bail fitted HRPD data

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 

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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