[GSAS-II] Parameters that won't refined

[Victor Zenou]

Hi

I'm working on Sc-doped TiO2 nano-crystallites. As Sc and Ti have close
atomic number, and therefore atomic scattering factor, XRD cannot be used
to differentiate between Sc and Ti, sitting on the same Wyckoff atomic
position. On the other hand, the neutron scattering length of Sc is
different from Ti.

Therefore, I'm using neutron diffraction (ND) data and X-ray diffraction
(XRD) data of a few Sc-doped TiO2 samples in a joint refinement using
GSAS-II. Among the usual lattice parameters and sample displacement (bragg
brentano geometry; XRD) or zero shift (*Debye**-*Scherrer ; ND) the
refinement includes Ti and Sc site occupancy parameters (s.o.f) in addition
to the crystallite size. These site occupancy parameters are refined at the
end, however they won't  be "refined" (refinement ends without changing the
s.o.f. parameters).

Changing these  Ti/ Sc s.o.f manually  can give better results. It is more
complicated as there are some vacancies at the same position.

Does anybody have an idea how to refine these s.o.f of Ti and Sc?

Best regards,

Victor Zenou
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