This discussion strikes a familiar cord with me. It seems quite reasonable
to expect that fitting problems will arise when sample broadening is very
small. For spallation tof, the issue in my mind is the ability to
precisely characterize the source contributions to the resolution
function. Issues of this kind were clearly seen when I built the GPPD at
IPNS. Though perhaps primative by HRPD standards, reconfigurations in the
source were clearly seen.
I recall also doing some work on X7A at NSLS on samples of nonlinear optic
materials for which the resolution function in GSAS fit the data, but the
physical significance of the parameters was in doubt. This was clearly a
case where the sample contribution to the resolution function was very
small and/or possibly pathological (in this case, the samples were ground
from highly perfect single crystals).
John
At 02:20 PM 9/23/99 +0100, you wrote:
>Thanks for all the suggestions.
>
>With regards to the following; the problem isn't that our sample is
>degrading the peak shape but rather that it isn't! We have an
>exceptionally crystalline sample and it seems that the GSAS peak-shape
>function only seems able to cope with the HRPD peak shape when convoluted
>with sufficient sample broadening. We've used GSAS for HRPD data analysis
>many times before with many different compounds/samples and not
>experienced this problem.
>
> > > We're forced to use CCSL for the superior peak shape description and
> > > so simply changing refinement program is not an option. I'm sure
> > > that this problem must have been encountered before and I'm suprised that
> > > there's not a 'standard' solution to it.
>
> > For HRPD data I'm surprised you can't get away with using GSAS unless
> > you have particularly nasty anisotropic peak shape effects.
>
>Eddie Cussen
>
>Inorganic Chemistry Laboratory,
>Department of Chemistry,
>University of Oxford,
>South Parks Road,
>Oxford, OX1 3QR
>United Kingdom
>E-mail: [EMAIL PROTECTED]
>tel: (..44)(0)1865-272602
>Fax: (..44)(0)1865-272690
_____________________________________
Dr. John Faber
Principal Scientist
ICDD
12 Campus Boulevard
Newtown Square, PA, 19073
ph : (610)325-9814
fax: (610)325-9823
e-mail: [EMAIL PROTECTED]
also see: http:\\www.icdd.com
