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
>
>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: [EMAIL PROTECTED]
