What are the important factors to select chebychev polynomial order for background function in TOPAS. If am checking the degree of crystallinity and I start with modelling the amorphous hump, do I select the order (minimum value) that best matches amorphous profile or any other factors are also to be considered. Does the value of 1/x background gives any indicaition for selecting the "order" value?
I have attached a screenshot to clarify my query
If you have a well crystalline sample with the same density and the same composition as the sample you want to evaluate, then you may 'learn' the background from this well crystalline sample. Measurement condition must be exactly the same for both samples. You may use a polynomial of any order, e.g. 20th order, to model the background of the well crystalline sample. Of course the the reflections should be modeled, too. Now fix th e background and replace the scan of the well crystaline with the scan of the sample containing an amorphous phase. The difference between fixed background (e.g. 20th order) and Bragg peaks may be assumed to be caused by the amorphous phase. The amorphous phase may be modeled by a peak phase or an hkl phase.
Rajeev, yes, minimum Chebychev (1) or this operation. But switch off the 1/x. This is only needed for very low angle background rising as in air scatter. Your pattern does not seem to show this, (at least at the magnification of this image) so best to turn it off. I know the tutorial tells you to switch it on but that pattern has a rising low angle background. (The tutorial often tells you to do things without really explaining why you are doing it - not a very good tutorial!! - Any Bruker people reading, take note!) Other things to check are the minimum settings on CryL to make sure it does not refine to a nonsensical figure. Also switch on calculate errors to check for high values.
Thanks a lot Ian, and I agree, in tutorials it would be of great help if Bruker add some information about such factors.
If you have a well crystalline sample with the same density and the same composition as the sample you want to evaluate, then you may 'learn' the background from this well crystalline sample. Measurement condition must be exactly the same for both samples. You may use a polynomial of any order, e.g. 20th order, to model the background of the well crystalline sample. Of course the the reflections should be modeled, too. Now fix th e background and replace the scan of the well crystaline with the scan of the sample containing an amorphous phase. The difference between fixed background (e.g. 20th order) and Bragg peaks may be assumed to be caused by the amorphous phase. The amorphous phase may be modeled by a peak phase or an hkl phase.
Thanks Robert for your detailed response. I am working on cementitious materials and the minimum number of crystalline phases that are formed are atleast 6. I did run pure crystalline sample of a single phase and when i compare the background of the pure crystalline sample with that of my hydrated cement paste sample containing both crystalline and amorphous phases, i see a big difference in low angle background i.e at less than 10deg 2theta. Pure crystalline material has very low background that less that 10 deg but that of cementitious materials is very high.
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