[Ifeffit] Inconsistency of the amplitude reduction factor

[Bruce Ravel]

On 09/18/2014 07:23 AM, Scott Calvin wrote:
> Hi Hoon,
>
> Using a reference value is not always a good idea, because experimental
> effects can play a role.

I want to elaborate a bit on this point in Scott's post, particularly 
given the nature of your sample.

The materials in a battery, as they charge or discharge, can undergo 
changes in morphology.  Morphology can have an impact on the measured 
amplitude.  More specifically, inhomogeneity in the sample -- pinholes, 
for instance -- have a known effect on the amplitude.  See, for example:

   http://dx.doi.org/10.1103/PhysRevB.23.3781
   http://dx.doi.org/10.1016/0167-5087(83)90730-5
   http://gbxafs.iit.edu/training/thickness_effects.pdf

My point is that you may need to question the assumption that S02 even 
should be transferable in your measurement.

B


> BUT, S02 should not change during charge-discharge on a single sample,
> or a series of samples prepared and measured similarly. Instead, it's
> likely something correlated with S02 in the fit is changing, and so the
> fitting routine is getting a bit confused and attributing part of the
> change to S02. (That's not a knock on the fitting routine; it doesn't
> know any better unless you tell it!)
>
> I think the best recommendation is to do a simultaneous fit on multiple
> spectra, constraining the S02 to be the same for each. So you're still
> fitting S02, but forcing all the spectra to use the same value.
>
> Second best is to fit one spectrum and allow S02 to vary, and then
> constrain all the other fits to use that value.
>
> --Scott Calvin
> Sarah Lawrence College
>
> On Sep 18, 2014, at 7:06 AM, HOON Kim <science at live.co.kr
> <mailto:science at live.co.kr>> wrote:
>
>> Hello,
>>
>> I am a bit confused about the amplitude reduction factor (S0^2), in a
>> sense that whether this factor must be determined by fitting or
>> constrained by a reference value for a specific element. I'm dealing
>> with a cathode composite (for lithium-ion battery) comprised of two
>> crystal phases. During charge-discharge, the amplitude reduction
>> factor changes and at a certain state of charge (SOC) it changes a lot
>>  such as from 0.77 to 0.67. My understanding is that it may reflect
>> the phase transition of the material into the amplitude reduction
>> factor. But, I'm not sure ... I need advice on this.
>>
>> Thank you !
>>
>> Kind Regards,
>> HOON
>> _______
>
>
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-- 
  Bruce Ravel  ------------------------------------ bravel at bnl.gov

  National Institute of Standards and Technology
  Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2
  Building 535A
  Upton NY, 11973

  Homepage:    http://bruceravel.github.io/home/
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