# Re: [Ifeffit] 4+2 model for distortion

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Hi Abdul:```
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Here is what I do under these circumstances, when the distances are very close.
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I first select one path only, set the value of N to the total number of neighbors (in your case 6) and then fir using only this one path. In your case, if you choose, F1, the initial value of N should be 4 and you change it to 6.
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I then look at the outcome of the fit, in particular, the value of sigma-squared. If this is physically reasonable, then I make the determination that it may be impossible to separate the two paths. So what is a physical value? Gnerally, if this is a room temperature measurement, I would expect a value of between 0.002 and 0.005 for a first shell Ni-F bond. If the value of sigma squared is higher than that, it could be legitimate to include the second path. In this case, I owul dset the value of N back to the original number (in your case 4) and fit again but constrain the sigma squareds of the two paths to be the same initially. Again, if the result is significantly better in terms of chi squared and the Real/Imaginary part of the data/fit and the sigma squared is physically reasonable, then I might decide to keep the two path fit. Remember that you wil have more free parameters and as such the chi-squared will usually be smaller for the two shell fit.
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The bottom line is that you need to decide what you are trying to extract from the data. Are you measuring the asymmetry of the octahedron as a function of temparature or some other parameter? If so, then perhaps using two paths is the right solution. basically, you need to try difrernt fits and then look at the results to make sure you have a good reason to choose one over the other. There is no magic bullet, I am afraid.
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Carlo

On Wed, 16 May 2018, Abdul Ahad wrote:

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```Hi Prof. Segre

I have used .cif file for the K2NiF4 type structure.
In this structure the first shell and first single path shows 6
coordination number. and the path is Ni to F1 with 2 leg. My concern is to
use 4+2 model. Do i change the CN number or i have to add one more site. I
cant visualize the situation. Becuase here the F have 2 sites i.e F1 and
F2. And the path is only for the F1. While the diffrence between the bonds
Ni-F1 and Ni-F2 is about 0.1 A. which lies under the resulution limit of my
experiment via delta r and delta k relation.​

*--------------------*
Junior Research Fellow
*Dept of Physics*
*Aligarh Muslim University*
*Aligarh 202002 (U.P.), India*

*http://www.researcherid.com/rid/G-8198-2018
<http://www.researcherid.com/rid/G-8198-2018>*
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On Wed, May 16, 2018 at 10:30 PM, <ifeffit-requ...@millenia.cars.aps.anl.gov
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```wrote:
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Today's Topics:

1. Re: 4+2 model for distortion (Carlo Segre)

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Message: 1
Date: Tue, 15 May 2018 16:00:08 -0500 (CDT)
From: Carlo Segre <se...@iit.edu>
To: XAFS Analysis using Ifeffit <ifeffit@millenia.cars.aps.anl.gov>
Subject: Re: [Ifeffit] 4+2 model for distortion
Message-ID: <alpine.deb.2.21.1805151558480.17...@hydride.segre.home>
Content-Type: text/plain; charset=US-ASCII; format=flowed

Hi Abdul:

Are you strating with a cif file for a particular structure or are you
simply working with a quick first shell theory?

Carlo

On Wed, 9 May 2018, Abdul Ahad wrote:

```
```hi all
How to include this model in atom that is 4+2 or 2+4 for 2short or 4 long
bond length of octahedra
thanks

*--------------------*
Junior Research Fellow
*Dept of Physics*
*Aligarh Muslim University*
*Aligarh 202002 (U.P.), India*

*http://www.researcherid.com/rid/G-8198-2018
<http://www.researcherid.com/rid/G-8198-2018>*
<a...@mailaps.org> *

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

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End of Ifeffit Digest, Vol 183, Issue 14
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--
Carlo U. Segre -- Duchossois Leadership Professor of Physics
Interim Chair, Department of Chemistry
Director, Center for Synchrotron Radiation Research and Instrumentation
Illinois Institute of Technology
Voice: 312.567.3498            Fax: 312.567.3494
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