PbSO4 Constant Wavelength Neutron Data
This problem was the subject of the IUCr Round Robin. The aim is to see how far one should take a Rietveld refinement of neutron diffraction data on a simple oxide. The data file is in pbso4_c.gsas and instrument file in pbso4.inst. An 1978 refined set of coordinates are in pbso4_92609.cif.
1. Read in the powder pattern and starting coordinates. Make sure that uiso values are sensible. You may need to manually change the space group to the non-standard setting of P b n m.
2. Go through a "standard" Rietveld refinement
protocol as described in lectures. You might initially want to refine over
a limited data range (e.g. up to 70 or 90 degrees).
3. Keep a note of how the R-factors decrease as you
add in extra parameters.
4. When you're happy with your refinement use e.g. atoms to view the structure and displacement parameters.
We got chi**2 of 2.8 and wRp=3.25% a pat on the back to anybody who beats that ..
Additional Work (optional)
Try using e.g. peak shape 1 and 4 in gsas to see if you can get a better fit. Try refining the data in topas.
[Modified 10-Apr-2021 by John S.O. Evans. Pages checked for Google Chrome.]