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Tutorials and Examples

LHPM-Rietica Rietveld for Win95/NT by Brett Hunter

Refining on Neutron Time of Flight data using Rietica-LHPM

The CCP14 Homepage is at http://www.ccp14.ac.uk

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

Much of the following is based on advice from Brett Hunter using Argonne TOF data. Mileage may vary depending on the nuances of your data.

It is assumed that you have the TOF Constants information:

  • incident spectrum information. (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)
  • Zero, Difc, Difa (d-spacing/peak position dependent terms)
  • alpha0 (normally alpha0=0), alpha1, beta0, beta1 (if you do not have these terms, they are possibly refinable depending on the quality of the data - which affect peak assymetry and other peak shape considerations)

The Incident Spectrum information is inserted into Rietica via the Model, Histograms menu then selecting the Incident Spectrum button

Inputting the Incident Spectrum values

The Zero, Difa and Difc constants are inserted into Rietica via the Model, Histograms menu. On a normal refinement, Difa and Difc would not be refined and would normally be obtained by running a standard such as NBS/NIST Silicon.

Model, Histograms menu

The alpha0, alpha1, beta0, beta1 constants are inserted via the Model, Sample menu.

Model, Sample menu

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.

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

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