fred Hi, I'm using th2_offset to account for displacements in my peaks as a function of 2theta using a polynomial: prm poly_b 0.00100`_0.00521 prm poly_c -0.00090`_0.03436 prm poly_d 0.15215`_0.13055 th2_offset = poly_b*((Th*180/Pi)^3) + poly_c*((Th*180/Pi)^2) + poly_d*((Th*180/Pi)); I'm finding this is working really well to improve my fit. I'm doing this on CeO2 and the reason I want to do this is so I can re-scale my 2Theta in all my other .xy files. However, I'm finding the polynomial coefficients that are output don't make much sense when I use them to calculate the new 2theta scale. Has anyone else had similar issues? Or is there something I'm missing?
johnsoevans My first comment is that you should be very careful with this type of approach. Peak shifts are caused by lots of factors and many of them are sample dependent (e.g. height, transparency, etc). They therefore can change with each measurement I would normally only try this type of approach when you have an internal standard (e.g. if using Si standard to calibrate 2th scale when the Si is mixed with your sample). If you're using the internal standard approach then you should write the equations in terms of Xo which describes the 2-theta scale and is in degrees (not radians). Th (radians) is reserved for hkl peak positions. See table 2.2 in Tech Ref. If you do find large discrepancies in observed and calculated peak positions then you should really check the diffractometer alignment to try and find their source. Hope that's some help.
fred Hi John, Thanks for the response. The reason I'm doing this is to correct some PDF data from the ESRF. The correction I'm trying to do is shown in this paper. Thanks for the suggestions, using Xo simplifies things.