[Ifeffit] Implementing constraints in EXAFS analysis

[Bruce Ravel]

On 09/05/2014 09:03 AM, Badari Rao wrote:
> I have collected EXAFS data on a ferroelectric perovskite Sodium Bismuth
> Titanate which has an average rhombohedral R3c structure and am trying
> to fit the data with R3c as the starting model. But I am having trouble
> in implementing constraints for the fit.
> I found a paper authored by you, titled "Local structrue and phase
> transitions of BaTiO3, Ferroelectrics, 1998, 206-207, pp 407-430". I
> want to do similar kind of analysis using Artemis on my data. However, I
> am finding difficulty in understanding how the constraints have been
> implemented for the refinement of the barium K edge EXAFS, especially
> those for the bond lengths (Page 416 in the above mentioned article in
> particular is unclear). Could you please explain how the GDS window and
> the variables for different paths in this case would look like. Also how
> do you decide what parameters to refine and what to keep fixed (table IV
> in the article)?
>
> Also can you please suggest some literature for beginners which can
> guide me in implementing such constraints?

Badari,

You are asking a very broad and open ended question.  Since you
self-identify as a beginner, I would recommend spending some time
learning the basics of XAS and XAS analysis.  You could start by
reading one of the recent text books on XAS by Grant Bunker or Scott
Calvin:

 
http://www.amazon.com/Introduction-XAFS-Practical-Absorption-Spectroscopy/dp/052176775X
   http://www.amazon.com/XAFS-Everyone-Scott-Calvin/dp/1439878633

To learn about the practice of XAS, there are links to tutorials at

   http://xafs.org/Tutorials

Shelly Kelly's tutorials specifically address the use of Athena and
Artemis.

To learn about the use of my software, there are links to a lot of
information, including links to videos of me demonstrating their use,
at the Demeter website:

   http://bruceravel.github.io/demeter/

To learn specifically about the application of constraints to EXAFS
analysis, two excellent papers with extensive discussions of how to
apply sophisticated constraints are:

   http://dx.doi.org/10.1103/PhysRevB.66.224405
   http://dx.doi.org/10.1016/S0016-7037(02)00947-X

While neither of those papers are on the topic of titanate
perovskites, you would do well to read them carefully and understand
the analysis strategies that Scott or Shelly describe.

HTH,
B


-- 
  Bruce Ravel  ------------------------------------ bravel at bnl.gov

  National Institute of Standards and Technology
  Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2
  Building 535A
  Upton NY, 11973

  Homepage:    http://bruceravel.github.io/home/
  Software:    https://github.com/bruceravel
  Demeter:     http://bruceravel.github.io/demeter/