HI Eugenio,

Have you seen this work? I came across it last night.
https://www.nature.com/articles/srep43791
"Local structures around the substituted elements in mixed layered oxides"

They experienced a similar issue:
 "... In the O3 compound, we included the contribution from the first- (O), second- (M), and third- (Na) nearest neighbor elements. In the P2 compounds, we included the contributions from the first- (O), third- (M), and fifth- (O) nearest neighbor elements. We omitted the contribution from the second- (Na2) and fourth- (Na1) nearest neighbor elements because the occupancy (g = 0.21–0.41) is small. In both cases, we fixed Nj for the three elements at the crystallographic value (=6)..."

"...We found that the R values of the third paths, i.e. the M–Na distances in the O3 compounds and the longer M - O distances of the P2 compounds, are seriously deviated from the crystallographic values. In addition, the σj2 values for these paths are significantly large (~0.01–0.03 Å2) as compared with those of the first and second paths. In this sense, the EXAFS analysis model including the third paths remains ambiguous. We performed another EXAFS analysis with omitting the third path to validate the discussion above. We observed the same trend of the M – O and M – M distances between the compounds even if we omitted the third path...."

Disorder does seem to be an issue. Do you have a pure phase result to which you can compare
i.e. unaltered or non-substituted?

cheers,
-R.





On 2024-01-16 11:40 p.m., Otal Eugenio wrote:
Dear Carlo,
Thanks for your answer.
Yes, it is a layered material. I was measuring in situ the transformation from O3 to P3. 
I am not sure that Na atoms are ordered, it is a light element in a layered structure and I applied some cycles to form the electrolyte-cathode interphase, and disorder can be found there. I shook the structure a little, your observation is correct.
I compared the spectra of a non-cycled one (blue) with a cycled one (red) and the differences are minimal.
image.pngimage.png
I was measuring all the edges, all showed the same problem, Na is impossible to include in the fitting.
The goal was to monitor changes in the first coordination shell during the battery operation. I found all the necessary information to understand the system. 
To fit the second coordination shell was attempted to evaluate the Na concentration along the cycles and complement the electrochemical measurements. 
Best regards.




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Eugenio H. OTAL
Assistant Professor
Dept. of Materials Chemistry
Shinshu University
4-17-1 Wakasato, Nagano 380-8553, JAPAN
eugenio_otal@shinshu-u.ac.jp
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On Wed, Jan 17, 2024 at 2:56 PM Carlo Segre <segre@iit.edu> wrote:
Hi Eugenio:

This looks like a layered structure, most likely a P2 or P3 phase.  Is this correct?  How sure are you that you have a single phase and that the Na ions are well ordered?  Is it possible that there is so much disorder in the M-Na distances that they wash out any contribution?  In that case, the second M-O distance would probably be more important.  You also do not mention if this is Ni, Fe or Mn edge data.  Is it possible that these differ?

Carlo



On Tue, Jan 16, 2024 at 11:30 PM Otal Eugenio <eugenio_otal@shinshu-u.ac.jp> wrote:
Dear Robert, 
Thanks for your detailed answer.
I am cutting at R=3.35. I also tried to go up to 4.0 to provide more freedom and to include the SS TM-O at 3.6.
It seems that including MS was not fruitful at the moment. I found the contribution very tiny, Importance < 10.
I included the path at 3.6, and it fits well, but when I include the Na, the CN_Na goes to high values (>10) and errors (~+/- 50)
I also tried to restrict the CN of TM to force the Na to take place and get good results. Also useless.
I was checking the data in R_real and the TM and Na paths. I see the TM path is in phase while the Na path is not. I wonder if this is affecting the fit. How should I constrain the fitting to solve this situation?
Best regards.

image.png



(^ㅇᆽㅇ^)(=˃ᆺ˂=)(=🝦 ༝ 🝦=)

Eugenio H. OTAL
Assistant Professor
Dept. of Materials Chemistry
Shinshu University
4-17-1 Wakasato, Nagano 380-8553, JAPAN
eugenio_otal@shinshu-u.ac.jp
https://sites.google.com/view/zettsu-laboratory/news-updates

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Who is John Galt?

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On Wed, Jan 17, 2024 at 1:24 AM Robert Gordon <ragordon@alumni.sfu.ca> wrote:
Hi Otal,

I ran a quick FEFF simulation just to see what is present under that second peak in the FT.
There are many more shells than just TM and Na...e.g.


There is significant multiple scattering and another backscattering peak (Fe-O) at
3.6Angstroms. All of these are going to spread underneath that peak in the FT. You might
also be getting some interference from the 4.07A MS stuff. It doesn't seem a surprise that
just including TM and Na is giving problems.

What is your high-R cutoff? Have you tried including multiple-scattering or that 4th backscattering
contribution?

cheers,
Robert

On 2024-01-09 12:06 a.m., Otal Eugenio wrote:
Dear all,
Happy New Year!
I am dealing with the fitting of NaMn0.3Fe0.4Ni0.3O2 second shell.
I have Mn, Ni, and Fe at 2.9772Å and Na at 3.1859Å, showing only one shell. If I use any of the transition metals (dZ <5) for fitting the second coordination shell. fitting is fine, but when I introduce the Na path, the fitting has no sense, CN_Na grows and also its error.
Even including Na, I have Nvary = 10 and Ninp = 13.98, so I am getting out of ideas and nothing helpful in the ifeffit Archive until now.
Is there any strategy for dealing with mixed second coordination shells? I am attaching the file exported from Larch and the cif file.
Best regards.


(^ㅇᆽㅇ^)(=˃ᆺ˂=)(=🝦 ༝ 🝦=)

Eugenio H. OTAL
Assistant Professor
Dept. of Materials Chemistry
Shinshu University
4-17-1 Wakasato, Nagano 380-8553, JAPAN
eugenio_otal@shinshu-u.ac.jp
https://sites.google.com/view/zettsu-laboratory/news-updates

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Who is John Galt?

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--
Carlo U. Segre (he/him) -- Duchossois Leadership Professor of Physics
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Illinois Institute of Technology
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