Combined Rietveld

Combined X-ray and Neutron – ZrW₂O₈

Files needed: zrwneut_sh.xy; 6g17pb01.raw

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 them using a template to build the combined refinement.. If you’ve done the earlier tutorials on ZrW₂O₈, 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  0.00157 y =Get(x);:0.00157 z =Get(x);:0.00157 occ  Zr+4  1.0 beq bxzr1  0.59130
site W1  x xw1   0.34115 y =Get(x);:0.34115 z =Get(x);:0.34115 occ  W+6   1.0 beq bxw1   0.84370
site W2  x xw2   0.60124 y =Get(x);:0.60124 z =Get(x);:0.60124 occ  W+6   1.0 beq bxw2   0.88803
site O1  x xo1   0.20824 y yo1  0.44030     z zo1  0.44771     occ  O-2   1.0 beq bxo1   1.67856
site O2  x xo2   0.78907 y yo2  0.57176     z zo2  0.55553     occ  O-2   1.0 beq bxo2   1.21253
site O3  x xo3   0.49472 y =Get(x);:0.49472 z =Get(x);:0.49472 occ  O-2   1.0 beq bxo3   2.07208
site O4  x xo4   0.23223 y =Get(x);:0.23223 z =Get(x);:0.23223 occ  O-2   1.0 beq bxo4   2.55131

5. You should be able to get Rwp ~7.9% after refining sensible parameters. To save typing a typical file for ZrW₂O₈ is linked here.

6. At the top of the INP TOPAS reports the overall r_wp. It’s useful to also look at the r_wp for each data set. Add “r_wp 0.0” under each xdd line. After refinement you’ll see the values are X-rays 7.6%, neutron 10.7%, overall 7.9%. The fact that the overall r_wp is closer to the X-ray value means that the weighting used is making the X-ray contribution more important. You could try adding the lines below to the neutron xdd section to increase its weighting. Assuming Poisson statistics this formula increases the weighting by a factor of 10. You should then get r_wp values of X 7.8%, N 10.2%, overall 9.1%. The fact that the overall r_wp is the average of the individual values suggests approximately equal weighting in the model.

	prm !neut_weight 1 'try 10 for roughly equal contributions
	weighting = If(Yobs < 1, 1, (1*neut_weight)/Yobs);

7. 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.

8. 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.

Combined X-ray and Neutron – ZrW2O8

Combined X-ray and Neutron – ZrW₂O₈