On Mon, 28 May 2007, Bruce Ravel wrote:
On Monday 28 May 2007, Joji Kuniya wrote:
I analyzed EXAFS spectra of trace of Fe in lithium tantalate crystals. At first, I created the feff.inp file of LiTaO3. Then I changed central Li atom to Fe atom and added Fe atom at the Ta site. So that, the fitted results is good except amp(amp results is 0.3). I think that the model structure is good, but I can not understand why amp is too low. How do you think about this results? And, I tried to fitting treatment using GDS parameters. But, I do not know that the general method how to use the GDS parameters. Please tell me the general way of GDS manipulation. My PC runs under Win XP, and My Artemis runs under feff8 conditions. Thank you. Joji Kuniya
Hi Joji,
You don't really provide enough information to give a good answer to your questions. I suspect that your problem has something to do with the fact that the amplitude and the various sigma^2 values are highly correlated. Without more information about how you are actually parameterizing your fits, though, it is hard to comment on that value of amp.
I am confused that you put Fe on both the Li and Ta sites. Is that actually reasonable? Could be -- but my hunch is that Fe would prefer to sit on the Ta site. Having the wrong element on a site can do bad things to a fit.
Hi Joji: I would agree with Bruce. In addition, I would suggest that if the Fe is only present in trace amounts, it is unlikley that there will be a lot of Fe around it. Rather it will be surrounded by O and either Li or Ta, depending on which site it subsititutes on. The first thing you need to figure out is where the Fe sits. This can be done by fitting the first shell only to the the two different environments surrounding the Li and Ta and seeing which works best (all you need to see is the distance and the number of O atoms to really distinguish). Once you have determined where things sit, you can proceed. You may not have an amplitude problem then. By the way, if you want to provide more information, you can attach your project file. There are usually a number of people who will just take a look and give you advice. Carlo -- Carlo U. Segre -- Professor of Physics Associate Dean for Special Projects, Graduate College Illinois Institute of Technology Voice: 312.567.3498 Fax: 312.567.3494 segre@iit.edu http://www.iit.edu/~segre segre@debian.org