Dear Rietvelders,
There is a question that keeps nagging me from time to time when i come across
negative thermal parameters in rietveld refinements (in this case neutron data).
Intensitiywise, Most variables can be classified as withdrawing or adding. In
the former group, we can find the thermal parameters, the profile coefficients
and the scale factor; in the latter, the absortion and extintion corrections
(discarding preferred orientation). The atom positions, lattice parameters and
diffactometer constants don't fit very well in any of these groups.
Small mistakes when it comes to fit the background are usually the causes of
such negatives values, when it is fitted as background what should belong to
peak intensity. Therefore, modifying background functions and number of
coefficients is usually enough to sort this problem out.
However, I have realised that GSAS assigns intensity with different priority to
withdrawing variables. The scale factor draws most of it, and if I keep
refining it, in some cases there is not enough intensity left for the thermal
parameters and, consequently, they go negative so that they add rather than
withdraw intensity in order to obtain the best fitting.
Would it be a valid strategy to turn the scale factor flag off at some point to
make sure positive thermal paramenters are obtained?
Regards,
Gerard
