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From: "C.A.Kirk" [C.A.Kirk@sheffield.ac.uk] To: rietveld_l@ill.fr Date: Thu, 30 Nov 2000 09:31:10 -0000 Subject: Combined XRD and ND refinements using GSAS Dear Rietvelders, I have a question concerning joint XRD and ND refinements using the prog. GSAS. The profile fit for the XRD part of the joint ref I am currently working on is quite bad and it seems to be due to a two theta error. The unit cell parameters from the joint refinement are only very slightly different to those I obtained from a refinement of the XRD data only: JOINT a=12.40855 c=3.93371 XRDonly a=12.41279 c=3.93499 I assume the values for the unit cell parameters obtained from the joint refinement are an average of those obtained from the XRD and ND datasets: is the weightin equal? Will such a small difference in the unit cell parameters be enough to cause the poor fit of the data? I calculated the two theta positions for both the unit cells and the first few reflections have differences of only, on average, 0.005. Any comments would be welcomed as well as suggestions on how to proceed. Thanks Caroline Caroline Kirk Solid State Research Group Department of Engineering Materials University of Sheffield Sir Robert Hadfield Building Mappin Street Sheffield S1 3JD Telephone: 0114 222 6013 E-mail: c.a.kirk@sheffield.ac.uk |
From: "Radaelli, PG (Paolo) " [P.G.Radaelli@rl.ac.uk] To: "'rietveld_l@ill.fr'" [rietveld_l@ill.fr] Subject: RE: Combined XRD and ND refinements using GSAS Date: Thu, 30 Nov 2000 09:55:50 -0000 Dear Caroline, in general, it is very difficult to calibrate a neutron diffractometer well enough to trust the absolute lattice parameters (the relative ones are usually excellent). This is especially true for TOF diffractometers. There are many reasons for this, including sample position and transparency effects. The latter are particularly annoying, because they are wavelength-dependent, and introduce a non-linear relation between TOF and d-spacing. Also, the asymmetric pulse shape does not help. En passant (are you listening Bob?), I notice that there is currently no Rietveld code I'm aware of capable of correcting neutron data for transparency. Ideally, the refineable parameter should be mu*R, exactly as for the absorption correction, and it should be possible to set a constraint between the two values. Also, a correction for sample position errors would be nice. Note that, for modern scintillator-based diffractometers, a 0.5mm positioning error (forward or backwards) is equivalent to a peak shift of 10% of the FWHM at 90 degrees. My usual advice to my users is that if they want to know the absolute lattice parameters, they should measure them on their x-ray diffractometer. Having done that, they should fix the lattice constants and refine the neutron instrument parameter constants for all banks on the room temperature data. These constants should than be fixed for the other temperatures. This advice should apply equally well to your case, especially if you have synchrotron data, which are usually very precise. The situation is not that bad for constant-wavelength diffractometers, especially of the multi-soller type (like D2B), provided that they are recalibrated every time the monochromator is moved. You should also consider the possibility that the difference you observe is due to a compositional gradient from the inside to the outside of the grain. Paolo Radaelli GEM instrument scientist |
From: "Hyatt, Neil" [NHyatt@lsm.co.uk] To: "'rietveld_l@ill.fr'" [rietveld_l@ill.fr] Subject: RE: Combined XRD and ND refinements using GSAS Date: Thu, 30 Nov 2000 11:13:38 -0000 Dear Caroline, I had a similar problem to yours caused by highly absorbing samples (particularly when using large diameter vanadium cans). Effectively the problem equates to a sample displacement error (I can give a suitably hand-wavy explanation if you like!) which translates into a shift in calculated unit-cell parameters. I employed the same strategy as Paolo Radaelli suggested... and refined the structure on the basis of "high resolution" Cu-Ka1 Lab data. I then fixed the cell constants in the neutron refinement and refined on DIFC in GSAS which essentially serves to get the tick marks in the right place. I then fixed DIFC and allowed the unit-cell constants to vary and proceeded with the refinement (I was doing high pressure stuff and so kept the refined value of DIFC fixed for non ambient pressures). This should also work for your joint neutron-xrd refinent. All the best, Neil Hyatt. |
Date: Tue, 05 Dec 2000 17:13:57 -0700 To: rietveld_l@ill.fr From: vondreele@lanl.gov (Bob Von Dreele) Subject: Re: Combined XRD and ND refinements using GSAS Dear Caroline (& others), Our practice here at LANSCE with TOF powder diffraction Rietveld refinements & multiple detectors is to use the highest resolution detector bank as the "reference" and then refine the lattice parameters and the diffractometer constants for all the other banks in a multihistogram GSAS refinement. The reference detector bank is in backscattering and is thus insensitive to some (but not all) sample positioning & absorption errors. This at least minimizes the systematic error. Usually only "DIFC" need be adjusted in these refinements. If you are also including an x-ray pattern with a well characterized wavelength (CuKa?) then use that for the reference & refine all the DIFC's for the neutron TOF data along with the lattice parameters. Don't forget to refine the usual systematic effect parameters for the x-ray data - most notably "shft"! Don't worry about instability - these things are pretty orthogonal to each other. I'd not refine DIFA or ZERO though, that will cause problems. The combined x-ray/neutron refinements do require that the experiments be done at the same temperature, otherwise the thermal expansion will mess up the combined refinement. This is real serious for noncubic materials. You probably don't have this problem as your c/a ratios were identical to about 5 parts in 300,000. By the way one can use the same trick for multiwavelength experiments done at a synchrotron; fix one wavelength (the lab CuKa one is best for this) and then refine all the other wavelengths and the lattice parameters together. Bob Von Dreele At 09:31 AM 11/30/00 +0000, you wrote: >Dear Rietvelders, > >I have a question concerning joint XRD and ND refinements using >the prog. GSAS. > >The profile fit for the XRD part of the joint ref I am currently working >on is quite bad and it seems to be due to a two theta error. The unit >cell parameters from the joint refinement are only very slightly >different to those I obtained from a refinement of the XRD data only: > JOINT a=12.40855 c=3.93371 > XRDonly a=12.41279 c=3.93499 >I assume the values for the unit cell parameters obtained from the >joint refinement are an average of those obtained from the XRD >and ND datasets: is the weightin equal? > >Will such a small difference in the unit cell parameters be enough to >cause the poor fit of the data? > >I calculated the two theta positions for both the unit cells and the first >few reflections have differences of only, on average, 0.005. > >Any comments would be welcomed as well as suggestions on how >to proceed. > >Thanks >Caroline |