> do you really have the
> resolution even on
> HRPD to see the diffuse scattering between Bragg peaks at
> high Q ?
No we don't, but this is not the main point (by the way, we don't use HRPD
for PDF, it doesn't go to sufficiently short wavelengths). The main reason
to go to high Q is to avoid
Bob,
This exactly what is needed when the sample is a mixture of amorphous
and crystalline components. But what happens when the material is a
single crystalline phase with some coherent defects? Don't the defect
<-> average structure correlations start to dominate, and separating
components is
> Two very good points by Armel:
>>all the very good PDF studies ...are made by using synchrotron data or
>>neutron data from spallation sources
And he is modest as well :-) But do you really have the resolution even on
HRPD to see the diffuse scattering between Bragg peaks at high Q ? You may
get
Two very good points by Armel:
>all the very good PDF studies ...are made by using synchrotron data or
neutron data from spallation
>sources
This is because they are the only means to get to high Q (i.e., high
resolution in real space) and sufficiently high resolution (in reciprocal
space) simult
Jon & others,
Well, there is an attempt at this in GSAS - the "diffuse scattering" functions for
fitting these contributions separate from the "background" functions. These things
have three forms related to the Debye equations formulated for glasses. The possibly
neat thing about them is that
>> I would argue that the Bragg &
>> diffuse scattering both reflect the average instantaneous atomic
>> structure.
>Yes. If you integrate over energy, the scattering function factors to a
>delta function in time, corresponding to an instantaneous snapshot of the
>spatial correlations. It is not a
Well, that is an old chestnut that Cooper and Rollet used to oppose to
Rietveld refinement. I think Rollet eventually agreed that Rietveld was
the better method. Has Bill really gone back on that ?
The difference between the two approaches are just an interchange of the
order of summations wit
Adding 2 cents to the discussion...
But I will try to convince myself otherwise :-)
Another reason which may preclude your self-convincing is the
fact that all the very good PDF studies of materials that are not perfectly
crystallized (producing diffuse scattering), though not being amorphous,
are
>> I would argue that the Bragg &
>> diffuse scattering both reflect the average instantaneous atomic
>> structure.
>Yes. If you integrate over energy, the scattering function factors to a
>delta function in time, corresponding to an instantaneous snapshot of the
>spatial correlations. It is not a
> Another reason which may preclude your self-convincing is the
> fact that all the very good PDF studies of materials... are
> not by using constant wavelength neutrons...
You are right Armel :-) About the current advantage of SR and TOF for PDF,
I mean. That is why I am interested in being convi
>> It is not a question of Bragg or diffuse scattering.
>
> Actually it is. Bragg scattering is equivalent to projecting all the
> scattering density in the crystal onto a single unit cell divided by the
> number of unit cells in the crystal and replicate that average unit
> cell. In other words,
>Yes. If you integrate over energy, the scattering function factors to a
>delta function in time, corresponding to an instantaneous snapshot of the
>spatial correlations. It is not a question of Bragg or diffuse scattering.
Actually it is. Bragg scattering is equivalent to projecting all the
scat
Dear Brian,
It is 10:30 pm here, and I am supposed to be taking a day off tomorrow to
be with my new grand-daughter :-) But I find PDF so interesting that I
can't resist replying.
> I would argue that the Bragg &
> diffuse scattering both reflect the average instantaneous atomic
> structure.
Yes
Alan,
But if you refine the full data with the same model, can there really be any fundamental difference, if in one case you simply do a Fourier transform to real space ?
in Rietveld refinement we throw away the non-Bragg peak data, where-as with PDF all scattering is included.
It is f
> PDF requires exquisite data and a true passion
for data analysis. If you have a good problem, you can get (probably) the
best PDF data worldwide "almost" routinely on my instrument GEM at the ISIS
facility (see also the cited paper by Billinge).
Paolo Radaelli
>
...on a final note, if you have
The only truly unique PDF information is about *correlations*. Let's say
you have two bonded sites, both with anisotropic thermal ellipsoids along
the bond, and let's assume that the motion is purely harmonic. A sharp PDF
peak will indicate that the atoms move predominanly in-phase, a broad PDF
p
Hi Alan (et al.)
So my questions are naturally:
1) Could not the Rietveld refinement also be extended to the full data
range ?
2) If that was done, would there be any fundamental difference between
the two methods of fitting ?
(Yes, I know they also refined on a restricted d-spacing interval
co
I'd only add that given the clue that the peak in GaInAs is split from the PDF then
one should model it that way in a Rietveld refinement. It should agree. The thrown
away info in a Rietveld refinement is also evident in the Bragg peak intensities -
shows up as "funny" thermal parameters, low at
>But if you refine the full data with the same model, can there really be any
>fundamental difference, if in one case you simply do a Fourier transform to real
>space ?
Thanks to Stefan Bruehne for providing an obvious (in retrospect :-) answer i.e. that
in Rietveld refinement we throw away th
I have long admired the Pair Distribution Function method as applied to
disordered crystal structures. I have of course known much longer about
the PDF method for liquid and amorphous materials - we have a short
wavelength neutron diffractometer specifically for that. I think Lachlan
recently rec
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