Tutorial Combined

Combined X-ray and Neutron – ZrW2O8

Files needed: zrwneut_sh.xy; 6g17pb01.raw; zx002.xye

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 of 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 then using a template to build the combined refinement.. If you’ve done tutorial 12 you’ll have everything you need.

3. From the TOPAS_Durham_Menus/Templates select X_N_combined_rietveld_template.inp. Save this to a new filename.inp.

4. Go through the file and replace the purple text after arrows with appropriate values.

For the X-ray data: change filename; specify Variable_Divergence_Intensity; monochromator angle zero, CuKa2 radiation.

For neutron data: 1.88507 wavelength.

For the structure, cell parameters all 9.155; space group P213, coordinates as below:

site Zr1 x xzr1 0.00174` y xzr1 0.00174` z xzr1 0.00174` occ Zr+4 1. beq bval1 0.42480`
site W1 x xw1 0.34124` y xw1 0.34124` z xw1 0.34124` occ W+6 1. beq bval2 0.70172`
site W2 x xw2 0.60118` y xw2 0.60118` z xw2 0.60118` occ W+6 1. beq bval3 0.76712`
site O1 x xo1 0.20754` y yo1 0.44126` z zo1 0.44864` occ O-2 1. beq bval4 1.66190`
site O2 x xo2 0.78967` y yo2 0.57200` z zo2 0.55561` occ O-2 1. beq bval5 1.05592`
site O3 x xo3 0.49416` y xo3 0.49416` z xo3 0.49416` occ O-2 1. beq bval6 1.97792`
site O4 x xo4 0.23147` y xo4 0.23147` z xo4 0.23147` occ O-2 1. beq bval7 2.50795`

5. You should be able to get Rwp ~8.8% after refining sensible parameters. To save typing a typical file for ZrW2O8 is linked here. .

6. 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.

7. 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. atch 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.