arbitrary scattering factor for Rietveld refinement of TOF NPD

piogun

Dear Topas users,
I am doing Rietveld refinement of a TOF neutron powder diffraction pattern of an inclusion compound that contains D2 molecules. I suspect that they are rotating freely in the crystal structure.
I have calculated scattering factor of such a freely rotating D2 molecule and would like to refine it using Topas. The problem is that I do not know how to input my scattering factor (even fitted with 5 exponentials like the scattering factors of atoms and ions for X-ray diffraction that are stored in the ATMSCAT.CPP file). I have created a fake isotope placed it in the isotopes.txt file, and input the scattering factor into the neutscat.cpp file but it seems that it does not work. Is what I want to do feasible at all with TOPAS?
I will be grateful for your help.
With kind regards,
Piotr Guńka

alan-coelho

Hi Piotr

To refine on the neutron scattering length use:

f0_f1_f11_atom f0 Ca @ 0.47

Search for f0_f1_f11_atom in the Tehcnical_Reference.PDF.

Note, for neutron data the occupancy is 100% correlated with f0. You could in fact refine on the occ parameter instead.

piogun

Dear Alan,
thank you for your suggestion. I am afraid this won't work since the scattering factor is not constant for a freely rotating molecule. The formula is C1 * sin(Q* C2) / Q, where C1 and C2 are some constants.
It can be fitted nicely with a f0[k] = c + [SUM a_iEXP(-b_i(k²)), i=1,5 like atomic scattering factor for X-rays.

Is there a way to input such a scattering factor for refinement against neutron data?