Dear Nick and GSAS

For refinement of silicon you need to make sure that you have the correct 
setting of the space group. If you download the silicon CIF from ICSD you may 
have the space group setting so that silicon atoms are on x = 0 y = 0 z = 0. 
You need to change this to the other setting so that silicon atoms are on x = 
0.125 y = 0.125 z = 0.125. This should give you a much better fit.

Best wishes

Dr. Tony Bell
Senior X-ray Technician
Materials and Research Engineering Institute
Sheffield Hallam University
City Campus
Howard Street
S1 1WB<>
0114 225 3401

From: [] On 
Behalf Of Nick Weadock
Sent: 30 August 2017 22:58
Subject: [GSAS-II] Help with refinement of laboratory xray data


I am attempting to refine diffractograms of the NIST SRM 640c Si standard that 
I acquired on an Inel CPS120 diffractometer. I was able to successfully get out 
the instrument parameters using peakfit, and the results are quite good (Rwp ~ 

However, I have no success when I try a refinement on the data.  The phase data 
comes from a CIF file, and the calculated reflection positions match fairly 
well with the peaks in the data. However, the "refined" pattern does not follow 
the data at all (Rwp > 54%) and the background is much larger than it should 
be.  I have tried several background functions and have varied the sample 
displacement parameters and the lattice parameter, but nothing helps.

Has anyone experienced a similar situation? Any advice is appreciated.

Nick Weadock
PhD Candidate, Materials Science
California Institute of Technology
1200 E California Blvd, Pasadena, CA 91125
MC 138-78
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