[GSAS-II] External: GSAS-II Digest, Vol 359, Issue 1

[Joel Reid]

Hi Kasa,

In my experience, in the absence of additional information (like morphology observations from SEM or TEM work), start with isotropic crystallite size and/or isotropic strain to see how well they fit the peaks. Unless you have very good data to high angles, you often only need to fit size or strain (they tend to be highly correlated).

If there seems to be anisotropic peak broadening that isn't well accounted for by an isotropic fit, try one of the anisotropic models. If your material is a high symmetry, mineral-type or inorganic phase (particularly with hexagonal symmetry, a good example is theophrastite, Ni(OH)2), you probably want to start with uniaxial size broadening (along 001 axis). A lot of these types of phases have needle or plate-like morphology that can cause much different broadening between the hk0 and hkl reflections. Visually observing how the anisotropic broadening varies with the peak hkl indices can help determine the appropriate axis, and give insight into what is going on.

With organics, I find the anisotropic strain model often (but not always) works better. Start with a small number of anisotropic terms and see whether the fit improves, and whether the refinement seems stable. 

If you don't have additional information from another technique, sometimes you just have to experiment with multiple anisotropic models and compare how well they improve the fit, whether they seem stable, and whether the refined parameters are highly correlated. But you should eventually want to choose either anisotropic size or anisotropic strain, not both (that's virtually always asking too much from the data).

For the size and strain results to be reasonably accurate, you need to have instrument parameters determined using a high quality fit to a standard with limited or no sample broadening (like NIST SRM LaB6) using the same instrument configuration. Even then, you are best to compare with another method if possible, like the SEM for crystallite morphology, to make sure both techniques yield consistent results.

The following paper has a lot of good information (J Appl. Cryst. 37, 2004, 911-924, Size-strain line broadening analysis of the ceria round-robin example). They also provide data sets from broadened and un-broadened ceria collected on different instruments, so you can try test refinements yourself. This example is isotropic, but it can also give you insight into whether you have obtained an appropriate fit to the unbroadened data set to obtain your instrument parameters.

Joel


Joel Reid, Ph.D.
Industrial Senior Scientist
Canadian Light Source
44 Innovation Boulevard
Saskatoon, SK
S7N 2V3

(306) 657-3854
www.lightsource.ca
http://www.usask.ca/maps/map-nav/campus_maps/uofsmap.pdf


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Today's Topics:

   1. GSAS-II size and microstrain model selection (kasa belachew)
   2. Crystallite size and microstrain (kasa belachew)


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Message: 1
Date: Thu, 19 Jan 2023 08:16:00 +0300
From: kasa belachew <kasabelachew at gmail.com>
To: gsas-ii at aps.anl.gov
Subject: [GSAS-II] GSAS-II size and microstrain model selection
Message-ID:
	<CAOMpuQ6OPhEq8CfJ5O_vYHRixbLYhcHPyTB4ubQp4fuYvMARLg at mail.gmail.com>
Content-Type: text/plain; charset="utf-8"

GSAS-II computes the crystallite sizes (  *10-6* m) in three different (isotropic, uniaxial and ellipsoidal) and Microstrains in terms of the amount of lattice spread, unitless fraction of ?d/d (or equivalently ?Q/Q) times 106 (isotropic, uniaxial and generalized) arrangement.
But, my question here is,
1. What is the criteria to select the model for my material?
2. Can we use all models for one material?
Thanks
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Message: 2
Date: Thu, 19 Jan 2023 10:00:49 +0300
From: kasa belachew <kasabelachew at gmail.com>
To: gsas-ii at aps.anl.gov
Subject: [GSAS-II] Crystallite size and microstrain
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	<CAOMpuQ6msc-x1eUeb=gJdvA-zuqZYS58xaWb-OXF46SV9VkEoA at mail.gmail.com>
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GSAS-II computes the crystallite sizes (  *10-6* m) in three different (isotropic, uniaxial and ellipsoidal) and Microstrains (isotropic, uniaxial and generalized) arrangement.

Q#. Why the value of Crystallite size and Microstrain becomes different with each different model?
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