[GSAS-II] Help with refinement of laboratory xray data

[Bell, Anthony]

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
Sheffield
S1 1WB

Anthony.Bell at shu.ac.uk<mailto:Anthony.Bell at shu.ac.uk>
0114 225 3401

From: gsas-ii-bounces at aps.anl.gov [mailto:gsas-ii-bounces at aps.anl.gov] On Behalf Of Nick Weadock
Sent: 30 August 2017 22:58
To: gsas-ii at aps.anl.gov
Subject: [GSAS-II] Help with refinement of laboratory xray data

Hi,

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 ~ 5%).

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.

Thanks,
--
Nick Weadock
PhD Candidate, Materials Science
California Institute of Technology
1200 E California Blvd, Pasadena, CA 91125
MC 138-78
x2520
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