resending, now typing more carefully: *** Hey Xianqin, If it is a (bulk) alloy system, and if A-A and A-B are 1NN pairs, then, unless there is a phase transition or other perturbations that can involve chemical changes either within the sample of heterogeneous (say, nanophase formation of different composition) or homogeneous (say, ordering changes from random solid solution to an intermetallic compound, as in Au3Cu etc.) nature, the coord. numbers should stay the same. Thus, it is not physical to vary them independently. Thus, if the coord. number of the A-1NN is 12 (as in fcc) then set NB=12-NA and worry only if NA comes out to be outside the range between 0 and 12. Anatoly -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov]On Behalf Of XQ WANG Sent: Thursday, August 31, 2006 2:54 PM To: ifeffit@millenia.cars.aps.anl.gov Subject: [Ifeffit] Alloy EXAFS data fitting Dear all, I am trying to understand some EXAFS data collected on an alloy system (sorry I can't say it since these are not my data). It is a two-element alloy system, name AB. Their fitting results showed that A-B coordination number and A-A coordination number did not follow the similar trend with the temperature increase. I am just wondering if this case is possible? I had thought if A-A and A-B are from the same compound, they should follow the same trend. Maybe I am wrong. Another question is whether it is OK to combine coordination of A-A in A compound with CN of A-A in AB alloy if these two A-A bond distances are very close? If this case is OK, then combination of A-A CN and CN of A-B could follow differnt trend (I guess). But I thought we could not combine CN from different materials. Maybe I am wrong again?? Please let me know any suggestions concerning this situation. Thanks! Xianqin _________________________________________________________________ Check the weather nationwide with MSN Search: Try it now! http://search.msn.com/results.aspx?q=weather&FORM=WLMTAG _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit

Thanks, Anatoly, Matt. This is metallic alloy system. I am thinking, if there is no phase changes and both CN of A-A and A-B increase with temperature increase, that would indicate the increase in nanosizes, as Anatoly hinted. If in AB alloy, A/B ratio is variable, but still in the same structure system, CN (A-A)+CN(A-B)=constant should be valid. This indicates the exisitence of atom replacement. If A/B ratio is variable, and the alloy structure also changes (phase transition happens), can we get something like: CN of A-B uniformly decreases, while CN of A-A decreases first and then increases as a function of temperature? (I guess this might be possible) When I read the figures, it was not specified that CNs were from different phases. I was confused. Thanks again, Anatoly, Matt. If my understanding was not quite right, please correct me. Have a great day, Xianqin

From: "Matt Newville" Reply-To: XAFS Analysis using Ifeffit To: "XAFS Analysis using Ifeffit" Subject: Re: [Ifeffit] Alloy EXAFS data fitting Date: Thu, 31 Aug 2006 14:55:50 -0500

Dear Xianqin,

Anatoly answered much of this, but I'll add a little more:

...

It is a two-element alloy system, name AB. Their fitting results showed that A-B coordination number and A-A coordination number did not follow the similar trend with the temperature increase. I am just wondering if this case is possible? I had thought if A-A and A-B are from the same compound, they should follow the same trend. Maybe I am wrong.

It's hard to say much without more system details (glass, crystal?), but it seems strange to see a temperature dependence of a coordination number. Most crystals would not have any change in CN with temperature except at structural phase transitions. As Anatoly said, there may be an exchange of one atom for another, but the total CN = CN(A-A) + CN(A-B) would be constant.

...

Another question is whether it is OK to combine coordination of A-A in A compound with CN of A-A in AB alloy if these two A-A bond distances are very close? If this case is OK, then combination of A-A CN and CN of A-B could follow differnt trend (I guess). But I thought we could not combine CN from different materials. Maybe I am wrong again??

I would say it's not OK to do this. The Ag-Ag coordination number from pure Ag (ie, 12), would not apply for Ag-Ag coordination in Au0.9Ag0.1, though the Ag-Ag bond distance would apply.

--Matt _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit

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