Sample-based peak assymmetry

nelson

Hi All, first post!

I have some data collected at ID31 at ESRF of a perovskite. We heated it up into the cubic phase, but unfortunately I'm having difficulty fitting the peak shape properly in a Pawley refinement in Topas. There's definitely some peak asymmetry (with larger tails to low 2 theta), however simple_axial_model doesn't fit it correctly. I think this is because axial divergence is not the origin of this asymmetry, and therefore the angular dependence of the asymmetry is not modelled correctly by simple_axial_model. You also see this if you artifically increase the simple axial model coefficient - the peaks at low angle are too asymmetric, and the peaks at high angle are still too symmetric.

I've tried putting in my own custom circles_conv instruction, but I'm essentially guessing functions, and I hoped someone had some idea of sample-based peak asymmetry (I'm thinking it must be strain?) and how best to include this in the model?

For information, I'm using the voigt peak shape function (i.e. lor/gauss_fwhm = p1*Tan(Th) + p2/Cos(Th)). Adding spherical harmonics to the lorenzian Tan(Th) term increases the goodness of fit quite considerably, so I'm prepared for the asymmetry term to include spherical harmonics as well).

I hope this makes sense!

Thanks!

ahhill

Hi nelson

Have you tried different parameters for the guassian and lorentzian components?

        prm pr1  0.007
        prm pr2  0.007
        prm pr3  0.007
        prm pr4  0.007
     
    Zero_Error(zero, 0.002)
    Simple_Axial_Model(axial, 1.07693`_0.04475)

        lor_fwhm = pr1 Tan(Th) + pr2/Cos(Th) ;
        gauss_fwhm = pr3 Tan(Th) + pr4/Cos(Th) ;

Adrian