On Sat, Mar 1, 2025 at 1:45 AM Robert Gordon via Ifeffit <ifeffit@millenia.cars.aps.anl.gov> wrote:
This Message Is From an External SenderThis message came from outside your organization.Hi B.ifeffit mailing list: https://millenia.cars.aps.anl.gov/mailman3/lists/ifeffit.millenia.cars.aps.anl.gov/
A correlation between formal oxidation and edge shift from metal (metal = pure covalent) to oxides
can certainly be observed. If you consider Fe bonded to O in a variety of compounds (i.e. not just Fe-oxides),
you will see that for different formal valence, the edge positions cluster. They are not all the same
for a given formal valence. This occurs due to changes in the Fe-O interaction when the O also interacts
with something else. The pre-edge can also show this. A particularly beautiful paper illustrating this is
by Petit, Farges, Wilke and Sole (J. Synchrotron Rad. (2001). 8, 952-954) for an assortment of Fe minerals.
For the LI2TiS3 compound in the plot you provided, I would take the edge position (XAFS Eo) to be
about 4975. I would want to look at the derivative and look at the first peak in the derivative of the
main rising edge (i.e., yes, max slope). Similarly for r-TiO2, I would consider the edge position to be
about 4978 eV. The difference in edge position between two Ti species with formal oxidation of +4
would be attributed to the differing bonding environments. With Ti-S more covalent than Ti-O, one
wold expect the Ti-S compound to have a smaller edge shift from the metal than the Ti-O compound.
"I am confused by this sentence:"The peak position for LTS-213 is approximately 4970 eV, which agrees well with that for rutile TiO₂ with a valence of titanium species in the 4+ oxidation state."
The authors observe peaks in close proximity for two compounds with differing bonding environments
(Ti-O vs Ti-S) and note that one of these compounds has Ti with a formal valence of +4.
What the authors have not done is a systematic study of correlations between pre-edge peak positions for
Ti-containing compounds where Ti is bonded to sulphur or oxygen. Any conclusions based on one
comparison are likely highly influenced by observer bias. They made a compound called Li2TiS3,
and they want to say S-2, Li+1 ergo Ti+4. They are being somewhat carefree in their interpretation
of the limited data. If they had at least compared to TiS2, it would be a more valid comparison.
Formal oxidation state is convenient for electron counting. For XAFS edge position, what influences
that is governed by the extent of charge transfer to/from the local environment about the absorber.
Correlations with formal oxidation state can be established for similar environments (i.e. ligand type),
but even that will show subtleties for variations in environment.
When analyzing and interpreting data, don't overreach.
-R.
On 2025-02-28 6:10 a.m., Benito Melas wrote:
Hi R.,
I understand that the pre-edge corresponds to a transition that is normally forbidden by symmetry, while the absorption edge represents the transition from the core level to the continuum.
In Fundamentals of EXAFS, Figure 7.3 shows that the absorption edge shifts when iron changes from its metallic state to +2 and +3 oxidation states. The caption states:"Fe K-edge XANES of Fe metal and several Fe oxides, showing a clear relationship between edge position and formal valence state."As the edge position is unclear for me in the LST compound, should it be the point of maximum slope?
The LST peak at 4975 eV could correspond to the B peak reported by Rossi et al. (DOI: 10.1103/PhysRevB.100.245207), which is an aspect I am not entirely familiar with. I reviewed the paper by Farges (DOI: 10.1103/PhysRevB.56.1809) and did not observe anything similar. This is the first time I have seen the B peak so intense; previously, I have encountered lower-energy peaks with higher intensity, likely due to distortions in the local symmetry.I understand that the pre-edge is more related to local symmetry than to the oxidation state, based on the manuscripts I mentioned. Could you please clarify this further?
I am confused by this sentence:
"The peak position for LTS-213 is approximately 4970 eV, which agrees well with that for rutile TiO₂ with a valence of titanium species in the 4+ oxidation state."
I will check the tutorials, I always catch something new.Best regards,
On Fri, Feb 28, 2025 at 2:04 AM Robert Gordon via Ifeffit <ifeffit@millenia.cars.aps.anl.gov> wrote:
This Message Is From an External SenderThis message came from outside your organization.Hi B.ifeffit mailing list: https://millenia.cars.aps.anl.gov/mailman3/lists/ifeffit.millenia.cars.aps.anl.gov/
Have you had any lessons on XAFS? i.e. attended an XAFS course offered by many synchrotrons and related
institutions.
Do you know the difference between pre-edge features and absorption edge?
In the plot you provided, the edge positions are clearly different.
Formal oxidation state is a convenient way to count electrons.
Edge position is governed by how the screening of the nucleus of the absorber is affected
by the type and arrangement of near neighbours - orbitals involved in bonding and extent of
charge transfer between absorber and ligands...hence the usefulness of XAFS as a technique
for characterising local environments.
Still confused? Start here: https://xafs.xrayabsorption.org/tutorials.html
-R.
On 2025-02-26 11:31 p.m., Benito Melas via Ifeffit wrote:
This Message Is From an External SenderThis message came from outside your organization.Hi all,I was following this article about Li2TiS3
and found this Ti edge XANES figure
The authors claim
The peak position for LTS-213 is approximately 4970 eV, which agrees well with that for rutile TiO2 with a valence of titanium species in the 4+ oxidation state.
Should we use the pre-edge to determine the oxidation state? Is it reliable, or should we use E₀ and compare it with references of well-known oxidation states?In addition, TiS2, in which titanium is also in the 4+ oxidation state, shows pre-edge peaks at 4968–4971 eV. (21) Considering the titanium oxidation state in the raw materials, the stability of the titanium species, and the XANES results, the oxidation state of the titanium species in LTS-213 is likely to be 4+.
The second conclusion is even more unclear.
Would you help me see if I am missing something in my understanding of oxidation states using XANES?
Best, B.
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