Solid AuCl EXAFS fitting – big N
Dear all, I have questions on the EXAFS fitting of a spectrum of solid AuCl. I find this quite important to resolve because AuCl is so widely used in the literature as a standard reference compound. I obtained the spectrum from the 2013 XAS workshop in Diamond Light Source. It is one of the reference spectrum in the cyanobacteria tutorial. The XANES in the spectrum looks similar to previously published XANES of AuCl. Lengke, M. F. et al., Can. J. Chem. 85: 651-659 (2007) Each Au in the compound has two Cl atoms attached to it, as seen in the crystal structure below. So I fitted the spectrum using two scattering paths: (i) the Au-Cl single scattering, and (ii) the forward through absorber (Au-Cl- -Cl-Au) multiple scattering path. In the fitting model, I fixed So2 (or amp) at 1. The variables that I used for the paths are: (i) N, drcl, sscl, dele; (ii) N = 2, drcl*2, sscl*2, dele. One would expect N to be 2, or perhaps even a little lower because amp = 1 is quite high, but instead, I get N = 2.35, significantly higher than expected. Reduced chi-square : 1706.6480592 R-factor : 0.0222459 guess parameters: N = 2.34968550 # +/- 0.18385678 [1.00000] dele = 0.82469594 # +/- 0.86953191 [0] drcl = -0.02909926 # +/- 0.00608062 [0] sscl = 0.00349927 # +/- 0.00072012 [0.00300] set parameters: amp = 1.00000000 Fixing the coordination number N at the expected value of 2 leads to unrealistically high amp values, as one would expect because of the strong correlation between the two parameters. Next, I tried adding the third and fourth cumulant terms to perhaps account for anharmonic vibrations. The variables for the two paths are now: (i) N, drcl, sscl, dele, third, fourth; (ii) N = 2, drcl*2, sscl*2, dele, third*2, fourth*2. amp was still fixed at 1. But that doesn’t seem to improve matters much, N is similar to the value found without cumulants. Reduced chi-square : 1114.1247832 R-factor : 0.0106796 guess parameters: N = 2.32392798 # +/- 0.21444748 [1.00000] dele = -2.54795042 # +/- 1.27479319 [0.82470] drcl = -0.07137580 # +/- 0.01429137 [-0.02910] sscl = 0.00338118 # +/- 0.00154424 [0.00300] third = -0.00056160 # +/- 0.00017873 [0] fourth = -0.00000155 # +/- 0.00003392 [0] set parameters: amp = 1.00000000 So I have two questions here: 1. Why is the EXAFS amplitude overall so high? The only possibility I can come up with is that the material is actually a mixture of AuCl3 and AuCl. The bond lengths are very similar and it is well known that AuCl is quite unstable in aqueous solution. I have seen the conversion of [AuCl2]- in solution to Au(III) in my own XAFS experiments. 2. I have tried to include a Au-Au first shell scattering path. But there doesn’t seem to be evidence for Au-Au scattering in the data. If the sample was pure crystalline AuCl, the first Au shell would be at 3.2 Å from the absorber, but I cannot fit such a path to the data. So I am wondering whether the AuCl spectrum is unreliable... Given how many times AuCl XANES has been published before, has anyone else perhaps run into this problem? Has anyone perhaps measured solid AuCl under exclusion of moisture and air, or several times as a function of time to assess beam damage? Does anyone perhaps have suggestions what else may explain the high EXAFS amplitudes and the absence of Au-Au scattering…? Attached are the Athena, Artemis and .cif files. I used Demeter 0.9.18 with perl 5.012003 and using Ifeffit 1.2.11d on Windows 7. Thank you for your attention. Best wishes, Sin-Yuen, Chang PhD Student School of Chemical Engineering and Analytical Science The University of Manchester
On 06/27/2014 01:14 AM, Sin Yuen Chang wrote:
Given how many times AuCl XANES has been published before, has anyone else perhaps run into this problem? Has anyone perhaps measured solid AuCl under exclusion of moisture and air, or several times as a function of time to assess beam damage?
I was under the impression that *everyone* runs into that problem. As you say Au(1+) chloride is quite unstable. Given that the white line in the data that you took from my LCF fitting example is so close in size to that of Au(3+) chloride, I would guess that the sample was no pure Au(1+) by the time we measured it. For something as tricky as Au(1+) chloride, I think it is unwise to blindly trust data that you found in some disreputable corner of the internet. It really seems like the kind of thing you need to measure for yourself if you want confidence in what you have. B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 Homepage: http://xafs.org/BruceRavel Software: https://github.com/bruceravel
Many thanks for the response, Bruce.
For something as tricky as Au(1+) chloride, I think it is unwise to blindly trust data that you found in some disreputable corner of the internet. It really seems like the kind of thing you need to measure for yourself if you want confidence in what you have.
Exactly what I am working on! Best regards, Sin-Yuen -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Bruce Ravel Sent: 27 June 2014 16:40 To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Solid AuCl EXAFS fitting – big N On 06/27/2014 01:14 AM, Sin Yuen Chang wrote:
Given how many times AuCl XANES has been published before, has anyone else perhaps run into this problem? Has anyone perhaps measured solid AuCl under exclusion of moisture and air, or several times as a function of time to assess beam damage?
I was under the impression that *everyone* runs into that problem. As you say Au(1+) chloride is quite unstable. Given that the white line in the data that you took from my LCF fitting example is so close in size to that of Au(3+) chloride, I would guess that the sample was no pure Au(1+) by the time we measured it. For something as tricky as Au(1+) chloride, I think it is unwise to blindly trust data that you found in some disreputable corner of the internet. It really seems like the kind of thing you need to measure for yourself if you want confidence in what you have. B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 Homepage: http://xafs.org/BruceRavel Software: https://github.com/bruceravel _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Dear Sin-Yuen, Your Au(I)Cl seems to be significantly oxidized to HAu(III)Cl4. It can be judged by the pronounced pre-edge feature at ~11921 eV. I added a few (likely good) spectra of Au(III) and Au(I) chloride reference compounds to your athena file (attached), together with the paper in which they were published. My AuCl(s) looks much less oxidized than yours, but still might be somewhat altered; the best Au(I)Cl2 reference, to my opinion, is the aqueous AuCl2(-) complex which is stable in acidic Cl solutions at elevated temperatures (included in the file). Please feel free to use these spectra by referring to the paper attached. best regards Gleb Dr. Gleb S. Pokrovski Directeur de Recherche CNRS Associate Editor of Economic Geology Associate Editor of Geochimica et Cosmochimica Acta Geosciences Environnement Toulouse (ex-LMTG) GET - UMR 5563 of CNRS, University of Toulouse III 14 avenue Edouard Belin , F- 31400 TOULOUSE, France e-mail: gleb.pokrovski@get.obs-mip.fr; glebounet@gmail.com phone: 33-5-61-33-26-18 , fax: 33-5-61-33-25-60 On 27/06/2014 20:11, Sin Yuen Chang wrote:
Many thanks for the response, Bruce.
For something as tricky as Au(1+) chloride, I think it is unwise to blindly trust data that you found in some disreputable corner of the internet. It really seems like the kind of thing you need to measure for yourself if you want confidence in what you have. Exactly what I am working on!
Best regards, Sin-Yuen
-----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Bruce Ravel Sent: 27 June 2014 16:40 To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Solid AuCl EXAFS fitting – big N
On 06/27/2014 01:14 AM, Sin Yuen Chang wrote:
Given how many times AuCl XANES has been published before, has anyone else perhaps run into this problem? Has anyone perhaps measured solid AuCl under exclusion of moisture and air, or several times as a function of time to assess beam damage?
I was under the impression that *everyone* runs into that problem. As you say Au(1+) chloride is quite unstable. Given that the white line in the data that you took from my LCF fitting example is so close in size to that of Au(3+) chloride, I would guess that the sample was no pure Au(1+) by the time we measured it.
For something as tricky as Au(1+) chloride, I think it is unwise to blindly trust data that you found in some disreputable corner of the internet. It really seems like the kind of thing you need to measure for yourself if you want confidence in what you have.
B
-- Bruce Ravel ------------------------------------ bravel@bnl.gov
National Institute of Standards and Technology Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973
Homepage: http://xafs.org/BruceRavel Software: https://github.com/bruceravel _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Dear Sin-Yuen, Your Au(I)Cl seems to be significantly oxidized to HAu(III)Cl4, as can be judged by the pronounced pre-edge feature at ~11921 eV, typical of square-coordinated Au(III) compounds. I added a few (likely good!) spectra of Au(III) and Au(I) chloride reference compounds to your athena file (attached), together with the paper in which they were published. My AuCl(s) looks much less oxidized than yours, but still might be somewhat altered; the best Au(I)Cl2 reference, to my opinion, is the aqueous AuCl2(-) complex, which is stable in acidic Cl solutions at elevated temperatures (included in the file). Please feel free to use these spectra by referring to this paper. best regards Gleb Dr. Gleb S. Pokrovski Directeur de Recherche CNRS Associate Editor of Economic Geology Associate Editor of Geochimica et Cosmochimica Acta Geosciences Environnement Toulouse (ex-LMTG) GET - UMR 5563 of CNRS, University of Toulouse III 14 avenue Edouard Belin , F- 31400 TOULOUSE, France e-mail:gleb.pokrovski@get.obs-mip.fr;glebounet@gmail.com phone: 33-5-61-33-26-18 , fax: 33-5-61-33-25-60 On 27/06/2014 20:11, Sin Yuen Chang wrote:
Many thanks for the response, Bruce.
For something as tricky as Au(1+) chloride, I think it is unwise to blindly trust data that you found in some disreputable corner of the internet. It really seems like the kind of thing you need to measure for yourself if you want confidence in what you have. Exactly what I am working on!
Best regards, Sin-Yuen
-----Original Message----- From:ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Bruce Ravel Sent: 27 June 2014 16:40 To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Solid AuCl EXAFS fitting – big N
On 06/27/2014 01:14 AM, Sin Yuen Chang wrote:
Given how many times AuCl XANES has been published before, has anyone else perhaps run into this problem? Has anyone perhaps measured solid AuCl under exclusion of moisture and air, or several times as a function of time to assess beam damage?
I was under the impression that *everyone* runs into that problem. As you say Au(1+) chloride is quite unstable. Given that the white line in the data that you took from my LCF fitting example is so close in size to that of Au(3+) chloride, I would guess that the sample was no pure Au(1+) by the time we measured it.
For something as tricky as Au(1+) chloride, I think it is unwise to blindly trust data that you found in some disreputable corner of the internet. It really seems like the kind of thing you need to measure for yourself if you want confidence in what you have.
B
-- Bruce Ravel ------------------------------------bravel@bnl.gov
National Institute of Standards and Technology Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973
Homepage:http://xafs.org/BruceRavel Software:https://github.com/bruceravel _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Dear Dr. Pokrovski, Thank you very much. Your data is very useful. And what a coincidence, I just found your paper a few days ago! Best wishes, Sin-Yuen From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Gleb Pokrovski Sent: 30 June 2014 08:18 To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Solid AuCl EXAFS fitting – big N Dear Sin-Yuen, Your Au(I)Cl seems to be significantly oxidized to HAu(III)Cl4. It can be judged by the pronounced pre-edge feature at ~11921 eV. I added a few (likely good) spectra of Au(III) and Au(I) chloride reference compounds to your athena file (attached), together with the paper in which they were published. My AuCl(s) looks much less oxidized than yours, but still might be somewhat altered; the best Au(I)Cl2 reference, to my opinion, is the aqueous AuCl2(-) complex which is stable in acidic Cl solutions at elevated temperatures (included in the file). Please feel free to use these spectra by referring to the paper attached. best regards Gleb Dr. Gleb S. Pokrovski Directeur de Recherche CNRS Associate Editor of Economic Geology Associate Editor of Geochimica et Cosmochimica Acta Geosciences Environnement Toulouse (ex-LMTG) GET - UMR 5563 of CNRS, University of Toulouse III 14 avenue Edouard Belin , F- 31400 TOULOUSE, France e-mail: gleb.pokrovski@get.obs-mip.frmailto:gleb.pokrovski@get.obs-mip.fr; glebounet@gmail.commailto:glebounet@gmail.com phone: 33-5-61-33-26-18 , fax: 33-5-61-33-25-60 On 27/06/2014 20:11, Sin Yuen Chang wrote: Many thanks for the response, Bruce. For something as tricky as Au(1+) chloride, I think it is unwise to blindly trust data that you found in some disreputable corner of the internet. It really seems like the kind of thing you need to measure for yourself if you want confidence in what you have. Exactly what I am working on! Best regards, Sin-Yuen -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.govmailto:ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Bruce Ravel Sent: 27 June 2014 16:40 To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Solid AuCl EXAFS fitting – big N On 06/27/2014 01:14 AM, Sin Yuen Chang wrote: Given how many times AuCl XANES has been published before, has anyone else perhaps run into this problem? Has anyone perhaps measured solid AuCl under exclusion of moisture and air, or several times as a function of time to assess beam damage? I was under the impression that *everyone* runs into that problem. As you say Au(1+) chloride is quite unstable. Given that the white line in the data that you took from my LCF fitting example is so close in size to that of Au(3+) chloride, I would guess that the sample was no pure Au(1+) by the time we measured it. For something as tricky as Au(1+) chloride, I think it is unwise to blindly trust data that you found in some disreputable corner of the internet. It really seems like the kind of thing you need to measure for yourself if you want confidence in what you have. B -- Bruce Ravel ------------------------------------ bravel@bnl.govmailto:bravel@bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 Homepage: http://xafs.org/BruceRavel Software: https://github.com/bruceravel _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.govmailto:Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.govmailto:Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
participants (3)
-
Bruce Ravel -
Gleb Pokrovski -
Sin Yuen Chang