Combined X-ray and Neutron - ZrW2O8
Learning Outcomes: This example shows a combined X-ray and neutron refinement of ZrW2O8. X-rays are more sensitive to metal coordinates, neutrons to oxygens. A properly weighted combined refinement can allow the most accurate refinements provided all systematic errors are correctly treated.
1. Save the files listed above to your working directory.
2. The process in setting up a combined refinement is essentially the same as when refining a single data set. There are certain parameters which relate to each individual data set (wavelength, zero point, background, etc, etc) and certain parameters common to both (e.g. atomic coordinates). When starting from scratch it's normally easiest to start with normal single data set refinements then combine the input files. To save typing a typical file for ZrW2O8 is linked here. Alternatively you can work from one of the templates as described below.
3. Open combined_riet_01.inp in jedit and look through the file. The various sections are labelled. Use the "Save/send to topas" icon to send the file to topas and then refine. You should get an Rwp of ~8.7%.
4. If you're feeling brave you could try structure solution using combined X-ray and neutron data. Try putting the line "Auto_T(10)" at the top of the file. Then try setting e.g. the O1/O2 coordinates to zero. Restrict the X-ray/neutron data maximum to e.g. 50 degrees 2-theta for speed. Run the file to try and find the oxygens. Try adding the line "Decompose(0.005)" on each data set for speed.
5. The file combined_solve_01.inp gives an idea of what's possible. All 4 oxygen positions are found from random starting points within a few cycles.
6. If you want to practice setting one of these files up from scratch then the easiest way is to use the Template/Select Template command and read in "X_N_combined_rietveld_template.inp". Save this file and change all the purple text.
[Modified 21-Mar-2018 by John S.O. Evans. Pages checked for Google Chrome.]