What is Rietveld refinement? Does "Rietveld" QPA exist?
Hill & Howard (1987) titled "QPA from Neutron Powder Diffraction Data
Using the Rietveld Method" and indeed they refined all structural
parameters of their simple phases in binary mixtures, a methodical
test of the approach.
However, the primary goal in QPA "refinement" is to reach an
acceptable fit without running into wrong scale factors. Therefore,
peak positions, profile shapes and all structural factors potentially
biasing the scale factors must be either refined or known previously.
If everything is known, just scale factors must be "refined", may be
by a linear optimization (published since the 1980ies for experimental
patterns), no "Rietveld" code is necessary.
IMHO such simple approach can work well only under very fortunate and
simple/well known circumstances.
However, in routine QPA especially of geological materials of unknown
origin (minerals with not always exactly known chemistry and
microstructure) we typically need to optimize, begining from any
published and selected structure models
- lattice parameters (always)
- profile parameters (always), often with complicated profile models
due to disorder, anisotropic line broadening, bimodal size/microstrain
distribution etc.
- most intensity relevant site occupation (Mg-Fe substitution, K
content of interlayers in micas...), sometimes constrained to lattice
parameters
- PO correction models (if necessary).
As commonly the atomic positions of minerals are often well known from
single crystal work and small deviations do not have a significant
impact on the total scale factors, the atomic coordinates are
typically fixed in QPA. As Debye-Waller factors are hard to refine
from diffraction patterns with limited angular range and for minor
components, it is necessary to fix them in routine work. This means
they must be known exactly (literature?, refined separately (if pure
material is available), or estimated.
So Alan, is the refinement of one site occupation factor in a QPA by
using a Rietveld software enough "structure" to label such analysis as
"Rietveld QPA"? Or, is refining cell parameters an acceptable criterion?
If not, any idea for a better name?
Greetings
Reinhard
Zitat von Alan W Hewat <alan.he...@neutronoptics.com>:
Yes, it's Rietveld Refinement if you refine the atomic structure
parameters. If you define the Rietveld method as simply the calculation
of
the powder pattern, that applies to just about any powder diffraction
study, and under values the importance of the method.
But you shouldn't refine structure parameters just to get a better fit in
QPA by Powder Profile Refinement. I don't know how often you find
entirely
new compounds in QPA, and can determine their structure well enough to
publish them as new compounds.
The R-factor in this study is impressively low, but it is meaningless as
Armel has shown.
________________________________
Dr Alan Hewat, NeutronOptics
Grenoble, FRANCE (from phone)
alan.he...@neutronoptics.com
+33.476984168 VAT:FR79499450856
http://NeutronOptics.com/hewat
_______________________________
On Mon, 15 Jan 2024, 18:01 Kurt Leinenweber, <ku...@asu.edu> wrote:
Hi, in our work we still sometimes encounter entirely new compounds with
simple structures like rocksalt, spinel, etc. in order to convince
ourselves and our readers that the materials are real, we have to fit
the
powder patterns - put in the atoms, refine lattice parameter(s), show
that
thermal parameters are reasonable, fit the peak shapes, background etc
all
together. In the case of spinels for example we can also refine the
single
atomic parameter and show that the bond lengths are reasonable. The
refinements are used to decide whether sites are fully occupied and to
decide the valence of the cation when multiple valence states are
possible.
It’s not the kind of thing that would get people really excited with the
crystallographic methods, but it is necessary for convincing ourselves
and
others that the identification of the compound is reasonable. Is that
Rietveld refinement? I don’t know so I call upon the list to say.
Kurt
Get Outlook for iOS <https://aka.ms/o0ukef>
------------------------------
*From:* rietveld_l-requ...@ill.fr <rietveld_l-requ...@ill.fr> on behalf
of Matthew Rowles <rowle...@gmail.com>
*Sent:* Sunday, January 14, 2024 11:04:25 PM
*To:* Alan W Hewat <alan.he...@neutronoptics.com>
*Cc:* Le Bail Armel <le-bail.ar...@orange.fr>; Rietveld_L <
Rietveld_L@ill.fr>
*Subject:* Re: Misconduct
I see the core contribution of the Rietveld method as the point-wise
calculation of a powder pattern. This then enables the refinement of the
parameters.
Yes, attributing QPA to Rietveld is incorrect; that is mainly due to
Hill/Howard and Bish/Howard - I try to make it a point to differentiate
this.
calling PDF refinements Rietveld refinements is just plain wrong
(afaik),
unless you actually are moving crystallographic parameters around, in
which
case, the nomenclature gets fuzzy..
On Sat, 13 Jan 2024 at 14:39, Alan W Hewat <
alan.he...@neutronoptics.com>
wrote:
The core of Rietveld refinement is REFINING the crystallographic
parameters to fit the pattern. That was the great innovation. Is that
done
here ? To calculate various patterns to refine phase composition you
necessarily need models of the crystal structures. But all refinements
of
powder patterns are not Rietveld Refinement. Quantitative analysis of
powder samples is an important technique, and to attribute that to
Rietveld
is wrong. Same goes for Pair Distribution Function analysis. Calling
everything "Rietveld Refinement" is not helpful, and actually hides the
fundamental contribution of this technique to crystallography.
________________________________
Dr Alan Hewat, NeutronOptics
Grenoble, FRANCE (from phone)
alan.he...@neutronoptics.com
+33.476984168 VAT:FR79499450856
http://NeutronOptics.com/hewat
<
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_______________________________
On Sat, 13 Jan 2024, 07:00 Matthew Rowles, <rowle...@gmail.com> wrote:
Hard disagree here.
You're using crystallographic parameters to calculate the (intensities
and
positions of the) pattern.
To me, that's the core of Rietveld refinement.
You need to apply a peak shape model. Why not apply a model from which
you
can extract crystallite parameters?
QPA can then be done on the output of a refinement, eg application of
the
internal standard method using the Hill /Howard algorithm.
Its still a Rietveld refinement, I'm not just fitting peaks willy nilly;
they're constrained by a crystal model.
Matthew
On Sat, 13 Jan 2024, 01:48 Alan W Hewat, <alan.he...@neutronoptics.com>
wrote:
Apart from the record number of atoms, phases, parameters, citations etc
used to describe just 3 broad peaks, I object to this kind of refinement
being called "Rietveld refinement". Luca called it "Rietveld-like" in
MAUD,
which was used here, but even that is wrong. The term "Rietveld
refinement"
should be restricted to the refinement of crystallographic parameters -
cell dimensions, atom coordinates etc. and not applied to quantitative
analysis of phases, particle size etc. Fitting peaks in powder patterns
was
done before Rietveld, who must be turning in his grave to see his name
associated with this kind of thing.
________________________________
Dr Alan Hewat, NeutronOptics
Grenoble, FRANCE (from phone)
alan.he...@neutronoptics.com
+33.476984168 VAT:FR79499450856
http://NeutronOptics.com/hewat
<
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_______________________________
On Fri, 12 Jan 2024, 12:57 Le Bail Armel, <le-bail.ar...@orange.fr>
wrote:
Hi,
A good candidate for the "worst Rietveld refinement of the year" award :
https://pubpeer.com/publications/12069A6AD9D5D34F26031F34705D06
<
https://urldefense.com/v3/__https://pubpeer.com/publications/12069A6AD9D5D34F26031F34705D06__;!!IKRxdwAv5BmarQ!bR8Hzbc4csqSqmzz4jXPaN2SAqzUu0gu8WOlXtZTOx8piJSkXhgF8JUGaWkbeE_zV8sgBuN71xsjjg$
Even the difference pattern is totally fabricated.
Best
Armel
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