ZrW2O8 X-ray and Combined X-ray/Neutron
Aim of tutorial: this tutorial will show you how one often needs to restrain M-O bond lengths using X-ray data, but can get better refinements by combining X-ray and neutron data in a single refinement. File 6G17PB01.gsas contains lab X-ray data for ZrW2O8 using an instrument that can be described by d5_kevex.inst.
1. Use a .cif file created from the neutron refinement as starting coordinates (if you've not done this try zrw2o8_rt.cif).
2. Refine the X-ray data initially fixing all atomic coordinates.
3. Use disagl to determine the W-O bond lengths.
4. Refine the coordinates using X-ray data.
5. Use disagl to see how much W-O bond lengths have changed.
6. Use restraints to keep W-O bond lengths at sensible values.
7. As exercise 9 or now you might try doing a combined neutron/X-ray refinement.
8. Try the same refinement in topas using 6g17pb01.raw.
X-rays aren't as good as neutrons for locating oxygens even with good data! Combined refinements are hard! You have to be 100% sure that there aren't systematic differences between data sets. If you're not careful a combined refinement can be worse than either single data set refinement. One also has to be very careful about including errors properly so the two data sets are correctly weighted.
[Modified 21-Mar-2018 by John S.O. Evans. Pages checked for Google Chrome.]