What you say is good to know.
Nonetheless, the type 4 profile model has built-in constraints according to the cell class -- so it is easy to use and refines with far greater stablility that the Lxx terms. My advice to non-experts is still the following:
If you suspect that you have some reflection classes having peak widths wider than others, try out the type 4 model -- refining the Sxx terms and then eta. If the fit improves significantly, you likely have anistotropic peak broadening. If not, you don't & go back to using the type 3 model and ignore the L tensor.
The Stephens model is correct only for strain broadening -- but my experience is that it does a pretty good job fitting other types of anisotropic broadening, for example anisotropic crystallite size broadening. In the cases where pretty good is not good enough, then it would make sense to check out the Lxx terms -- but only after trying the Type 4 model to confirm that the observed broading indeed does vary by reflection class.
Brian
Andreas Leineweber wrote:
Dear Brian,
recently it was pointed out (J. Appl. Cryst. 37 (2004) 123-135) that the approach proposed by Von Dreele can have indeed - under certain circumstances and restrictions to the parameters - a physical meaning, e.g. concentration fluctuations can show up like this, and the type of anisotropy constitutes a special case of the Stephens model.
Best regards
Andreas Leineweber
Brian H. Toby wrote:
My advice on this question is that one should not use this approach. The Stephens formalism, coded in profile type 4, is better founded by theory. See the GSAS manual and reference to Peter Stephen's J. Appl. Cryst paper from a few years back.
Brian
Christophe Chabanier wrote:
Hello everybody,
i have a question about the GSAS software. Indeed, i would like to know what are exactly the L11, L22, L33....L23 parameters. I saw that these parameters represent the anisotropic microstrain in material. Moreover, there is an empirical expression which uses these parameters as following :
Gamma(L) = L11*h^2 + L22*k^2 + L33*l^2 + 2*L12*hk + 2*L13*hl + 2*L23*kl
I would like to know and understand the physical representation of these parameters and this expression.
Thanks in advance
Christophe Chabanier INRS-�nergie, Mat�riaux et T�l�communications 1650 Blvd. Lionel Boulet C. P. 1020, Varennes Qc, Canada J3X 1S2
_T�l:_ (450) 929 8220 _Fax:_ (450) 929 8102
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