Re: [Ifeffit] Vanadium-Chromium Prussian Blue Analog XANES fitting

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 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 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 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 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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