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Stephens anistropic strain parameters
LuciaMaini #1
Member since Nov 2016 · 3 posts · Location: Bologna
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Subject: Stephens anistropic strain parameters
Dear all,
 I'm refining a structure in the space group R3m described with an hexagonal cell.

 Since TCHZ peak shape is not optimal ( some peaks are broad and some are narrow) I tried the Stephens anistropic strain
 I adpated the macro  for the Trigonal for hexagonal indeces following the article
 P. W. Stephens, J. Appl. Crystallogr., 1999, 32, 281–289
  the macro I wrote is this one:

macro Stephens_trigonal_R3 (s400,s202,s004,s301,eta)
prm mhkl = H^4 s400 + K^4 s400 + L^4 s004 +
3 ( H^2 K^2 3 s400 + H^2 L^2 s202 + K^2 L^2 3 s400) +
2 ( H K^3 2 s400 + H^3 L s301 + K L^3 2 s400)+
3 ( H^2 K L 3/2 s301 - H K^2 L 3/2 s301 + H K L^2 s202);

prm pp = D_spacing^2 * Sqrt(Max(mhkl,0)) / 1000;

 gauss_fwhm = 1.8/3.1415927 pp (1-eta) Tan(Th) + 0.0001;
 lor_fwhm = 1.8/3.1415927 pp eta Tan(Th) + 0.0001;
I hope it is correct ! it works ( no syntax errors)
But I have problem  to define the parameter s400,s202,s004,s301

 I started with an equal value for all parameters (prm=100) and let them refining or should I get the starting value from somewhere?
are negative parameters acceptables?

Thank you
AlanCoelho #2
Member since Aug 2011 · 302 posts
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To define s400 for example the use:
  prm s400 #start min #min max #max

where #start, #min, #max are numbers corresponding to the starting, minimum expected value and maximum expected value.

Also, eee the section in the Technical Reference how to define a parameter
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