Hi Scott, Thank you for pointing out at Vince's work on ferrites. It is a nice demonstration of the effect of site occupancy by dopants on EXAFS. In Vince's APL 68, 2082 (1996), they found the distribution of Zn between A and B sites (octahedral and tetrahedral) by including MS, as you described. However, he varied only one parameter in the fit to the MS range - site occupancy, keeping all other fixed (p. 2083). As such, it is not a non-linear least square fit, but a linear least square fit of a (linear) combination of fixed functions, and thus the chi-squared of such fit must have a single minimum - either within the range of x, or at the boundary (x=0 or 1, as they obtained in most cases). What Tadej is describing seems more like a general, non-linear least squares fit problem, that has N minima in chi squared of which N-1 are false. It is counter-intuitive to assume that the contributions of EXAFS from each inequivalent Nb can be fixed in the fit except for its fractional occupancy. What about sigma2 of different MS paths? They can be equal in certain cases of collinear focusing paths, or you can use other constraints, e.g., correlated debye model to calculate them ab initio and constrain in the fit, but the former case is very rare for a multi-site system, and the latter approach (correlated debye model) is very difficult to test for such a complex system before you make sure that you can use these calculations in such fits. Thus, you obviously reduce the number of variables but you may exclude the true, physical, minimum of chi squared from your parameter space. Anatoly -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov]On Behalf Of Scott Calvin Sent: Saturday, December 17, 2005 5:29 PM To: XAFS Analysis using Ifeffit Subject: RE: [Ifeffit] my problem Hi Tadej, I agree with the general drift of Anatoly's comment--this is what Shelly meant by saying it sounds like a tough problem! But I think it came off sounding a little more harsh than I suspect Anatoly meant it to be, particularly considering some of the systems he himself has examined! Anatoly's description is fair IF only a nearest-neighbor fit is performed. But it's possible that inequivalent sites have very different environments at greater Reff, particularly since niobium is so much heavier than oxygen! For example, there's been a lot of work done that shows that there is a screamingly large difference in the EXAFS spectra of A and B sites in the spinel structure (Vince Harris did a lot of this, for example). In such a case, Artemis is able to find the site occupancy without too much trouble, using a strategy such as Shelly explained. Of course, =seven= freely-floating sites sounds likely to yield nonsense fits. It would help if you could use some physical intuition to try to simplify the problem, at least at first. Consider also using the "restraint" feature in Artemis/Ifeffit liberally in your early fits to keep the fit from wandering off into nonsensical territory. --Scott Calvin Sarah Lawrence College At 12:14 PM 12/15/2005, Anatoly wrote:
boundary="----_=_NextPart_001_01C6019A.F6711239"
You cannot resolve these sites from EXAFS analysis. Even if you had only 2 inequivalent sites of the same central atom with occupancies N1 and N2, you would not be able to resolve N1 and N2 in the fit because different Nb atoms would have similar environments (e.g., 4 and 6 oxygens), and they will have similar Nb-O distances around each site. The best you can do is to try to fit the first nearest neighbor peak as an average Nb-O contribution (if it is a well isolated peak) and try to make use of sigma2 that may turn out to be larger than in some well known Nb oxide reference that you should analyze first.
Anatoly
Dear collegues,
my name is Tadej Rojac and I'm writing from the Institute Jozef Stefan in Ljubljana, Slovenia. I work in the Department of electronic ceramics. Actually, I'm working on a EXAFS spectra with Artemis. What I am trying to do is mainly to describe an EXAFS spectrum with a model concerning the structure of Nb2O5. I'm working togheter with prof. Iztok Arcon, who is a specialist in the field. My main problem is that I have to do some suppositions. In fact Nb2O5 structure is quite complicated. It is composed of seven different Nb positions which I took into account using 7 feffs in Artemis. I don't know the exact occupancies of the seven positions. In order to make the search easier I did at first a supposition that all the occupancies must be positive, which is realistic. I set this with the command "abs" in each feff file. Secondly, I need to supose that the sum of all occupancies is 1. In that way the search for the result is much easier, otherwise I get the sum greater than one, which, for sure, is not the case. My question is how can I do that? For example, if the occupancies are O1, O2, O3, O4, O5, O6 and O7 and I define them with a starting value (let use say the same, so 1/7) than I can do O = O1+O2+O3+O4+O5+O6+O7. Finally I need to do O =1, but here Artemis doesn't want to define the same parameter (O in this case) twice. So, how can I insert the conditions, that the sum of all the occupancies is 1, into Artemis? I hope you can help me.
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