[Ifeffit] Vanadium-Chromium Prussian Blue Analog XANES fitting
newville at cars.uchicago.edu
Thu Jul 30 21:51:36 CDT 2015
On Thu, Jul 30, 2015 at 6:38 PM, Adora Graham <abaldwin2242 at gmail.com>
> 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.
>From a quick look at the V spectrum, it looks much more reduced than 4+ to
me -- more reduced than 3+, even. At least when compared to the metal
oxide series. How were you doing the comparison?
It can be challenging to extrapolate from XANES with different ligands, and
Prussian Blue with linear CN ligands might act quite differently from O
ligands. Like, I wouldn't necessarily suspect the VS04 was a particularly
good reference for you, though I could be wrong.
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?
Generally XRF or mass spec (if surface sensitive is OK XPS) is preferred
for elemental concentrations over XAFS edge jumps. To use XAFS edge steps,
you'd have to know the thickness of the sample to high precision, and ought
to correct for everything else in the beam. Transferring that between
different edges would be another complication, though V to Cr is pretty
close. Were the edge jump data on different edges of the sample piece of
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