Multiphase Rietveld Refinement
Learning Outcomes: This example shows a simple multiphase Rietveld refinement using laboratory data. It's a slightly artificial example that we use for training people to perform qualitative phase identification in Durham. Rietveld refinement allows one to attempt quantitative analysis on the same data set. Beware: don't trust absolute numbers unless you're working very carefully and have performed all the necessary data corrections and independent calibrations!
Topas advantages: quick, easy and robust; phases can be refined independently or parameters linked between phases in a straightforward manner.
1. Save the datafile and cif files in your working directory.
2. The input file can be set up in exactly the same way as a single phase Rietveld refinement, one just has to load in 3 cif files - one for each phase.
3. Click on "Select Data File" and navigate to find the file d5_05628.raw.
4. In Instrument/Corrections select "Durham_d5000" - the diffractometer used to collect the data.
5. Click on "Refine zero point" to flag that zero should be refined.
6. In "Structure - cif" click on the icon to "Read a CIF file" and select y2o3.cif. Repeat for wo3.cif and si.cif. You may find it convenient to give each phase a name. Either type "phase_name y2o3" on the line after "str" for each structure in the input file or place the cursor after "str" and select "Phase Name" from the miscellaneous menu.
7. Click the icon to send the input file across to topas then refine.
8. The refinement should converge rapidly to Rwp ~ 25.8%. Topas will give an estimate of the relative quantity of each phase on the Rietveld plot.
9. Try refining the cell parameter of each phase. Remember to give a/b/c identical parameter names for the cubic Y2O3/Si; don't refine alpha/gamma for monoclinic WO3. You should get Rwp~20.1%.
10. Refine a single temperature factor for each phase. Using column editting in jedit type e.g. "beq by2o3 1" at the end of each atomic site line for Y2O3. Repeat for WO3 and Si. You should now get Rwp~16.9%.
11. Look at the fit. It's clear that the single peakshape being used for each phase isn't modelling peaks shapes very well. Try giving each phase a different set of peakshape parameters. This can be done by changing the variable names for the peak shape for each phase. e.g. change the Y2O3 peakshape line to (i.e. add "1" after each parameter name):
TCHZ_Peak_Type(pku1,-0.03017`,pkv1, 0.04196`,pkw1, -0.01404`,!pkx1, 0.0000,pky1, 0.13006`,!pkz1, 0.0000)
You should now get Rwp~12.95%. Note that since peak asymmetry is being described by the "Simple_Axial_Model", which is instrument specific; it doesn't make sense to refine a different value for each phase.
12. This is a very simple example of a multiphase refinement. There are lots more clever things you could do. Play!
13. In case you mess up try the file linked here.
[Modified 31-Jul-2017 by John S.O. Evans. Pages checked for Google Chrome.]