Use of bkg_file() macro for Complex Background in Pawley vs Rietveld

jmas5

Hi All,

I'm currently trying to fit a dataset, where I've gotten good pawley fits, but am struggling to make the Rietveld work. I think this is an issue with my .inp, as opposed to a scientific one, as the structure is known and I have fit it just fine before. I have gotten good Rietveld refinements on ex-situ data of the same material.

In this case, I am working on operando data, with a significant background (it is the bulk of the pattern). I have a background scan for this (cell with everything except material of interest), and using bkg_file() in the case of the pawley fitting, I am able to a good fit, with the background scaling appropriately.

When I move to a Rietveld refinement, it is like I am unable to refine the scale of the bkg_file and the scale of the calculated pattern independently. I understand why this would not be an issue for the Pawley fit, since the peak intensities are arbitrary and no phase scale is refined. Using Rietveld, I am still able to get a good fit of the background to the data, but in doing so the scale of the phase itself gets very small. If I manually set the scale higher, you can see that the phase's peaks are in the correct place, but the intensity of the background scan becomes enormous (increasing exponentially with 2theta). I'm wondering if there is perhaps something linked in my input file that I don't realise, or can't find, as I've not had any issues using this macro previously.

I've included some screenshots below for reference:

Pawley fit:

Letting the phase scale and background refine, minimises phase:

Setting phase scale to fixed value, background still refining (start vs end):

I've attached the .inp file in question in case this is useful. For the purpose of trouble shooting/trying to set this up, I am only using bank 2 in this case.

shareable-op3-riet-test.txt

124kB

Any advice or suggestions greatly appreciated!

jmas5

Looking at it, I am actually getting a good fit to the phase at higher Q, so I'm wondering if potentially there's some issue with scaling calculated intensity as a function of Q? This is TOF data, I would not be surprised if it was something like this (but unfortunately I've not been able to spot it myself). Thanks again!

jmas5

Further thoughts from a colleague, neither of us are too sure how we would investigate this in the .inp so any advice appreciated:

Having a quick think about it, I wonder if it's got to do with the way in which both scales (background are phase) are being related mathematically in the input file. It's almost like they are not being treated separately as when they are both refined, the scale of the background seems to be more than enough to account for the overall scale not needing the scale to be even present. The opposite occurs when forcing the scale to stay fixed; the scale of the background can't go down potentially due to some mathematical constraint imposed by the phase's scale.