Simple Lab Rietveld Refinement – TiO2
Files needed: d5_05005.raw; rutile.cif
Learning Outcomes: This example shows a simple Rietveld refinement of an inorganic material using laboratory data. It introduces the use of menus etc in jedit.
Topas advantages: quick, easy and robust; all parameters can normally be refined simultaneously even when they’re a long way from their true values.
1. Save the datafile and cif file in your working directory.
2. Work through the menus in the Rietveld refinement section of Durham jedit menus.
3. Click on “Select Data File” and navigate to find the file d5_05005.raw.
4. In Instrument/Corrections select “Durham_d5000_scint” – 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 rutile.cif.
7. Click the icon to send the input file across to topas (see examples tutorial). You’ll be prompted for a name to save the input file to. The default of d5_05005_riet_01.inp is fine.
8. Click the icon to launch topas if it’s not already running.
9. Click on the “run” icon in topas.
10. The refinement should converge rapidly to Rwp ~ 16%.
11. Try refining temperature factors (e.g. type “beq @ 1” at the end of each “site” line containing atomic information. Try refining cell parameters by introducing parameter names. i.e. change the cell parameter lines to read:
a lpa 4.594 b lpa 4.594 c lpc 2.959
12. You should get a Rwp of ~ 15.5%. RBragg will be around 4.2%
13. There’s one free fractional coordinate in this structure – the x parameter of O. Try refining this by changing the oxygen line to read:
site O1 x xo1 0.30600 y xo1 0.30600 z 0.00000 occ O 1.00000 beq @ -0.0896`
Here both x and y coordinates are given the same parameter name which forces them to be identical. Alternatively you could change the O line to read as below. Here the y coordinate is determined by a simple topas equation. The “:0” after the equation is optional – it gets topas to write the outcome of the equation in the input file for you to check.
site O1 x xo1 0.30600 y =Get(x);:0.0 z 0.00000 occ O 1.00000 beq @ -0.0896`
14. You might like to try the influence of different peak shapes, different numbers of background terms, refining a sample height correction instead of a zero point, etc. Try changing the parameter axial to values of 0 then 20 and fixing it (change parameter name to !axial) to see the influence of axial divergence on peak shape.
15. Are you in a true minimum? Try adding the lines below to the “General Information about refinement” section of the file to randomise the structure according to esd’s on parameters: