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.
-----Original Message-----
From: C.A.Kirk [mailto:[EMAIL PROTECTED]]
Sent: Thursday, November 30, 2000 9:31 AM
To: [EMAIL PROTECTED]
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: [EMAIL PROTECTED]
