[GSAS-II] Calculating Particle size and Strain in GSAS2

[Toby, Brian H.]

Shoaib,

Let me answer your e-mail as best as I can.

First of all, you must accurately determine the instrument response terms for the specific instrument that you used and as set up for your experiment to get accurate site/microstress estimates. Remember that you are measuring the extra broadening from your sample, so you must know what your instrument is doing before you can know what broadening is “extra”.

On Jul 14, 2014, at 11:51 PM, Shoaib Muhammad <mshoaibce at gmail.com<mailto:mshoaibce at gmail.com>> wrote:

it. Instrument parameter file was supposed to be used in Fullprof and has the following text:

“Myresolution function
  0.000030   0.000050   0.000110 0 0 0”

I am not sure how to convert widths but I know this is covered in "Typical values of Rietveld instrument profile coefficients” by James A. Kaduk and Joel Reid, Powder Diffraction<http://journals.cambridge.org/action/displayJournal?jid=PDJ> / Volume 26 / Issue 01 / March 2011, pp 88-93 (http://dx.doi.org/10.1154/1.3548128). I would still recommend collecting and fitting data with a standard [NIST SRM 660(-,a,b) is a good choice, but anything with minimal broadening would do] collected under the same conditions as your sample.

 1-      In old GSAS there were couple of Peak profile function like “Pseudo Voith” and may be Pearson VII also. How can I select/refine these peak Profile functions in GSAS2?

GSAS-II has only one profile function and it works. It is a Pseudo-Voigt (sum of Gaussian & Lorentzian; a real Voigt is the convolution, BTW).

2-      Is it correct to fix U,V,W, X,Y in GSAS2 in-order to calculate “Crystal Size” and “Microstrain”?

Yes, you must know the first 5 terms.

3-      Can I refine “Crystal Size” and “Microstrain” together?

Yes, if needed and your range of data is large enough. Note that GSAS-II allows for either or both to be anisotropic with either a preferred direction (recommended) or a full expansion.

4-      What does it mean by LGmin check box in Profile/Data tab?

Microstrain (aka residual stress) and crystallite broadening is usually Lorentzian (LGmin = 1.0) but can have a Gaussian component (LGmin=0 is pure Gaussian). You can refine this and see if the result is reasonable (0 <= LGmin <= 1) and gives a better fit.

5-      How to export the strain data to plot the image? I selected Mustrain plot type, I can see the preview in plot window but can’t export data. If Mustrain is selected and I run the refinement, GSAS2 quits and I can’t open this file again. It always say “Python stopped working”.

The strain data are the coefficients you get, but you should be able to export the plot if you want. Please provide a lot more details on your computer and exactly what you are doing, as well as a GPX file if you want us to debug this.

Brian




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