[GSAS-II] GSAS-II Model selection

[Toby, Brian H.]

On Jan 22, 2023, at 6:59 AM, kasa belachew via GSAS-II <gsas-ii at aps.anl.gov<mailto:gsas-ii at aps.anl.gov>> wrote:

It is so good, that, GSAS-II computes the crystallite sizes (  *10-6* m) in three different (isotropic, uniaxial and ellipsoidal) and Microstrains (isotropic, uniaxial and generalized) arrangement.
Q1# Can we use more than one model for the same material?

Are you asking, can one use (for example) isotropic and ellipsoidal crystallite size together? That is not possible and would not work if it were allowed, since they would be completely correlated.

If you are asking about mixing different models for crystallite size and microstrain? Yes, that definitely possible and it makes complete physical sense that a material with nearly isotropic crystallite sizes could have highly anisotropic microstrains (or the opposite). But, see next comment on most datasets being insufficient for doing this.

Q2# Why the value became different with different model of the same material?

I answered this before in my previous reply. This is because crystallite size and microstrain (and possibly background) tend to be quite correlated and most datasets can only be used for an accurate fit to one of the two broadening types. When you are overfitting there are many possible results that give about the same fit. In that case, the results are pretty arbitrary and changing one set of parameters causes the others to change. To fit both microstrain and crystallite broadening together, you need clearly resolved peaks at high Q, say >90 deg with Cu Kalpha. Even then it is only possible if both types of broadening are significantly larger than the instrumental broadening. Clearly high-resolution synchrotron and neutron measurements will be much better for this.

Brian

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