GSAS2 Y2O3 Demo
GSAS-2 Y2O3 Demo Data
Files needed: y2o3_start.cif, y2o3_demo.xye, y2o3_demo_instprm.txt, y2o3_demo_gpx.txt
Learning outcomes: how to do a simple Rietveld fit of a Y2O3 pattern as discussed in the lecture.
1. download the .cif file, the .xye file and instrument parameter file y2o3_demo_instprm.txt. Rename y2o3_demo_instprm.txt as y2o3_demon.instprm.
2. Launch GSAS-2. Go to Import/Phase and read in a starting structural model from Y2O3_start.cif.
3. Go to Import/Powder pattern, choose format “from Topas xye” and select the data file y2o3_demo.xye, use instrument parameter file y2o3_demo.instprm (it’s in gsas-2 format). In the dialogue window that appears tick to add the Y2O3 to the powder pattern. Save the project file as *.prj.
4. Try and do a standard Rietveld fit. In “Controls” set to do 10 cycles of least squares. In Limits you might find it easiest to initially set the 2-theta limits to 2 to 60 until you have a reasonable model. In Background select a 6th order Chebychev. You will need to specify the instrument as being “Bragg Brentano” in the “Sample Parameters ” sectionand refine a specimen displacement parameter.
5. Go to Calculate/Refine to perform the Rietveld refinement. View the fit in the pattern viewer.
6. One Rietveld strategy might be to refine: (a) scale, (b) add in the cell parameter, (c) add in sample height, (d) add fractional coordinates on Y’s, (e) add uiso on Y’s, (f) refine and isotropic size and strain broadening on Y2O3, (g) refine O fractional coordinates and temperature factor, (h) extend range to 150 degrees. You should get Rwp ~14.5% or lower.
7. Test features like preferred orientation and a Stephens/Popa hkl-dependent peak shape model to see if they are significant. Try excluding any spikes in the data. Try refining the instrumental peak shapes. I got Rwp 12.25 and chi2 1.44 at this point.
8. If you get stuck save y2o3_demo_gpx.txt as y2o3_demo.gpx. This is a “working refinement” in a gsas project file.