Paolo,
On Fri, 15 Sep 2000, Radaelli, PG (Paolo) wrote:
.....
> Finally, here is a question for Bob and Juan. To me, it would be much more
> natural to remove Vo from the scale factor, that is to redefine a new S' so
> that
>
> Y=S'*L*A*E*|F|^2/Vo^2 and S'=K*Ltot*f
>
> This way, the scale factor will only depend on the sample effective density
> and not its crystal structure. This is very useful in phase transitions
> involving a change in the size of the unit cell, as you can imagine.
> Is there any rationale in doing it the way it's currently done?
[Admittedly it wasn't a question for me, so sorry for butting in.]
Here are two unit cells and scale factors fitted to the same (x-ray)
dataset using PRODD. I'm assuming this is what was meant? The cell doubles
and the scale stays the same (roughly). I'd be pretty confident it does
the same with TOF and CN data provided it's a recent version.
C 8.392690 8.392690 16.775499 89.83088 89.83088 89.80757
L SCAL 91322.
C 8.392508 8.392508 8.387020 89.83144 89.83144 89.80786
L SCAL 91479.
Suffice to say the use of cell volume in scale factors within CCSL was
rather vague until we tried to quantify a multiphase sample. I think a
factor was in for lab x-ray data which still doesn't work anyway, but not
for TOF or CW neutrons. The rationale for doing it this way was that I
weighed the multiphase sample myself and this was the only way to get
sensible numbers out. GSAS got the weight fractions right as well, so the
volume factor must be in there somewhere...
Does this mean GEM now produces data where histogram scale factors may be
constrained to be equal across all banks? An impressive achievement, but
it opens a can of worms with sample absorbtion and attenuation. It has
been suggested that absorbtion corrections are something which belong
firmly with the data, as they depend on the geometry, size and packing
density of the sample (packing density can be measured). As such the A*
values could be supplied to the program, and applied to the model. Bruce
was sorting this out for PRODD. Any comments on this idea? It has been
rumoured that absorbtion (for neutrons at least) is not a refinable
quantity, but something that ought to be known(!)
Best wishes,
Jon
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