Refining the Cell Parameter for a Cubic Powder Pattern

Files needed: none or data set in cubic_peaks.pdf; y2o3_peak_positions.xls

Learning outcomes: How to refine the cell parameter of a material in excel.

1. In the morning tutorials you'll have indexed the powder pattern of Y2O3 by hand.

2. The same exercise is set up in y2o3_peak_positions.xls as sheet "index_peaks". Take a look at this and check your work from the earlier tutorial.

3. In sheet "refine_cell" the spreadsheet has been set up to predict peak positions from the unit cell parameter and hkl indices. You can then use solver to minimise the discrepancies between observed and calculated d-spacing or 2-theta values.

4. Try minimising the difference in obs/calc d-spacing values. What value do you get for the cell parameter?

5. Try minimising the difference in obs/calc 2-theta values. What value do you get for the cell parameter?

6. Would you choose to do 4 or 5?

7. Sheet refine_cell_zero also includes a zero point in the refinement. Try minimising the difference in obs/calc 2-theta values. How does the plot of differences in observed/calculated 2-theta values compare to the one you got in 5? How does your cell parameter change?

8. Try doing a Pawley refinement of the data set d5_00451.raw over the same 2-theta range. If you've not done a Pawley refinement before then use the input file linked d5_00451_pawley_01.inp.How does the cell parameter compare to your value in 5?

9. Try doing the Pawley refinement but allowing the parameter axial to refine from a starting value of 5 to model peak asymmetry. What happens to the wRp value? What happens to the cell.

10. Extra work: try indexing the same pattern using topas autoindexing routines.

[Modified 27-Oct-2019 by John S.O. Evans. Pages checked for Google Chrome.]