Re: Rietveld Neophyte

[Leonid Solovyov]

Dear Dave,

Actually, GSAS is not the best choice for crystal size analysis. There are 
specialized programs for this purpose such as MAUD for instance.

If you can’t refine the Gaussian broadening parameters GU GV and GW you may get 
them from the refinement for a standard with big (>1000 nm) crystallites in 
order to allow for the instrumental broadening. Then you should refine not only 
LX, but also LY parameter responsible for the strain broadening. If you are 
lucky enough to refine LX without strong correlations with LY, background and 
other parameters, you may get an estimation of the crystallite size from the 
expression given in chapter ‘Interpretation of CW powder profile coefficients’ 
of GSAS manual. However, for “horrible data with very broad peaks of very low 
intensity” you may get misleading results due to incorrect separation of size, 
strain, and instrumental broadening contributions.

Best regards,
Leonid

***
 Leonid A. Solovyov
 Institute of Chemistry and Chemical Technology
 660049, K. Marx 42, Krasnoyarsk, Russia
 www.icct.ru/eng/content/persons/Sol_LA
 sites.google.com/site/solovyovleonid
***

--- On Fri, 10/23/09, David Weston  wrote:

> From: David Weston 
> Subject: Rietveld Neophyte
> To: Rietveld_l@ill.fr
> Date: Friday, October 23, 2009, 10:27 AM
> 
> Hello,
> 
> I have been using the GSAS software to analyse some XRD
> data from MgH2
> samples that have been heavily milled.  
> One piece of information I would like to assertain is the
> mean particle
> size of my powders.  The powders are of the order of
> 10 nm by TEM.  
> Some relevant information;
> I am using a type 2 profile function.
> My data is horrible with very broad peaks of very low
> intensity.  
> I have had to set the GU GV and GW parameters in order to
> do anything at
> all. If I let them float then I lose any semblance.  
> I get a Chi2 < 1.
> 
> My question is this;
>     How do I obtyain the particle
> size?  
> 
> Is it the LX parameter and is this measured in Angstroms?
> 
> I hope someone will be kind enough to help me out of this
> mire!
> 
> Regards
> 
> Dave Weston 
> Research Fellow
> Department of M3 
> Wolfson Building
> University of Nottingham
> 



  

Rietveld Neophyte

[David Weston]

Hello,

I have been using the GSAS software to analyse some XRD data from MgH2
samples that have been heavily milled.  
One piece of information I would like to assertain is the mean particle
size of my powders.  The powders are of the order of 10 nm by TEM.  
Some relevant information;
I am using a type 2 profile function.
My data is horrible with very broad peaks of very low intensity.  
I have had to set the GU GV and GW parameters in order to do anything at
all. If I let them float then I lose any semblance.  
I get a Chi2 < 1.

My question is this;
How do I obtyain the particle size?  

Is it the LX parameter and is this measured in Angstroms?

I hope someone will be kind enough to help me out of this mire!

Regards

Dave Weston 
Research Fellow
Department of M3 
Wolfson Building
University of Nottingham

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