[Ifeffit] Vanadium-Chromium Prussian Blue Analog XANES fitting

Carlo Segre segre at iit.edu
Fri Jul 31 12:33:40 CDT 2015


Hi Adora:

FEFF9 does a pretty good job of calculating XANES now.  YOu might want to 
try using that to confirm what you think you have.  I agree with you that 
having an environment similar to your target compound is essential for 
calibration of valence.

Carlo

On Fri, 31 Jul 2015, Adora Graham wrote:

> Chris,
>
> It looks like your other vanadium papers all seem to involve vanadium
> oxides in different oxidation states. While I think this method works very
> well for metal oxides, in my case there should only be isocyanides bound,
> from the vanadium perspective (not counting defect sites), so I'm not fully
> convinced whether or not this method is applicable here. What are your
> thoughts?
>
> Thanks,
> Adora
>
> On Fri, Jul 31, 2015 at 4:36 AM, Chris Patridge <patridge at buffalo.edu>
> wrote:
>
>> If you have Dr. Whittaker-Brooks data then you have the work we did on
>> vanadium oxides. You can look at some of the other vanadium papers we did.
>>
>> Chris
>>
>> Sent from my iPhone
>>
>> On Jul 30, 2015, at 11:49 PM, Adora Graham <abaldwin2242 at gmail.com> wrote:
>>
>> Chris,
>>
>> I have attached a copy of the correlation plot that I am using. I could
>> also send more information as to how I am picking the peaks (which might
>> also be the problem here), but that would take significantly more time to
>> generate at this point.
>>
>> I was using the main edge values reported in Wong's paper (in red), since
>> one of my standards (VSO4) does not have a pre-edge peak, so it wouldn't be
>> comparable. The main edge value I get for VOSO4 differs by ~0.6eV compared
>> to Wong's reported value, so I think I'm adjusting things correctly. I
>> actually have access to the raw data from J. Mater. Chem., 2011, 21,
>> 5580  (with permission from Dr. Whittaker-Brooks), which I am using as
>> several of my standards (in black).
>>
>> I have a feeling that my main issue is the difference in attached atoms
>> (CN vs. O) of my compound compared to the standards. What do you think?
>>
>> Regards,
>> Adora
>>
>> On Thu, Jul 30, 2015 at 6:12 PM, Chris Patridge <patridge at buffalo.edu>
>> wrote:
>>
>>> The standard curve using V standard, how are you making the curve?  You
>>> probably know from the Wong paper that there are pre edge features for the
>>> vanadium oxide species. We used a very similar method to assign oxidation
>>> states to doped vanadium oxide compounds between 3-5. The method gave us
>>> results that were consistent with other evidence.
>>>
>>> Chris
>>>
>>> Sent from my iPhone
>>>
>>> On Jul 30, 2015, at 7:38 PM, Adora Graham <abaldwin2242 at gmail.com> wrote:
>>>
>>> Hello,
>>>
>>> First of all, I am still very new to XAS analysis, so please excuse any
>>> misunderstandings I may have.
>>>
>>> I have a series of compounds nominally consisting of KxVx[Cr(CN)6]
>>> Prussian blue analogs (where x is ~1) with Cr(III) and presumably V(II)
>>> present in the system. From other evidence, the V(II) may be oxidizing to
>>> V(III). I have XAS data collected at the V and Cr K-edges from APS and have
>>> been struggling through trying to analyze my data using Athena.
>>>
>>> I am trying to get two things out of this data:
>>>
>>> First, I would like to get the oxidation state of the Vanadium sites. If
>>> they are not V(II), to get the ratio of V(II) to V(III) present, if
>>> possible. I have been trying to follow a similar analysis to Wong (Phys.
>>> Rev. B, 1984, 30, 10, 5596-5610). However, I seem to be running into the
>>> issue of finding standards with a similar ligand set to my sample. I have
>>> XAS data for the following standards: VO, V2O3, V2O4, V2O5, V(acac)3,
>>> VO(acac), VOSO4, and VSO4. I start with VSO4 as my initial vanadium source,
>>> so that could be useful for a comparison. Our initial idea was to use the
>>> vanadium oxides to give a standard curve to then extrapolate the oxidation
>>> state of my compound, but using this gives me an oxidation state of ~4.2
>>> which seems unreasonable for my system. I am kind of lost otherwise as to
>>> what else I could do with this.
>>>
>>> Second, I would like to get the ratio of V to Cr. I was told I could
>>> compare the un-normalized K-edge heights of the vanadium to that of the
>>> chromium to get this ratio. This doesn't seem right, as I would think that
>>> there should be some relative intensity factor involved. Also, this gives
>>> me a fairly unreasonable value of 4:1, since XPS measurements give me
>>> values of around 1:1.
>>>
>>> Can you point me in the right direction for these issues?
>>>
>>> I've also attached the project files for one of my samples.
>>>
>>> Thanks,
>>> Adora
>>>
>>> <AGB02-083-Cr merged.prj>
>>>
>>> <AGB02-083-V merged.prj>
>>>
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>> <XAS main edge correlations.pdf>
>>
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>

-- 
Carlo U. Segre -- Duchossois Leadership Professor of Physics
Director, Center for Synchrotron Radiation Research and Instrumentation
Illinois Institute of Technology
Voice: 312.567.3498            Fax: 312.567.3494
segre at iit.edu   http://phys.iit.edu/~segre   segre at debian.org



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