OK, thanks Cora. Yes, an internal standard using a simple well characterised material is a good idea. I perhaps over reacted when you listed all the parameters you refined for a multi-phase sample, including ADPs. ________________________________ Dr Alan Hewat, NeutronOptics Grenoble, FRANCE (from phone) alan.he...@neutronoptics.com +33.476984168 VAT:FR79499450856 http://NeutronOptics.com/hewat _______________________________
On Tue, 16 Jan 2024, 18:05 Lind-Kovacs, Cora, <cora.l...@utoledo.edu> wrote: > I should have been more specific – this was not with respect to QPA > specifically – it was a more general statement about the terminology of > when we should or shouldn’t use the expression “Rietveld refinement”. > Although we actually often do Rietveld refinements on 2 phase mixtures, > where one phase is an internal standard, which we use to determine sample > height offset changes with temperature. > > And no, we don’t just try to get lower R-values by randomly adding > parameters! 😉 > > Cora > > > > *From:* Alan W Hewat <alan.he...@neutronoptics.com> > *Sent:* Tuesday, January 16, 2024 11:54 AM > *To:* Lind-Kovacs, Cora <cora.l...@utoledo.edu> > *Cc:* rietveld_l@ill.fr > *Subject:* Re: [EXTERNAL] Re: Misconduct > > > > > > "In many cases, we will indeed refine those structural parameters to get > the best possible match". > > > > The best possible match is not the objective of QPA, but rather the best > possible estimation of the phase fractions. You can refine parameters > describing particle size, strain, lattice constants, etc for each phase, > but please believe the structural coordinates obtained from well > characterised (usually mono-phase) samples. > > > > Whether it is structure refinement or phase refinement, please resist the > temptation to add parameters simply to obtain a lower R-factor. > > > > ________________________________ > Dr Alan Hewat, NeutronOptics > Grenoble, FRANCE (from phone) > alan.he...@neutronoptics.com > +33.476984168 VAT:FR79499450856 > http://NeutronOptics.com/hewat <http://neutronoptics.com/hewat> > _______________________________ > > > > > On Tue, 16 Jan 2024, 17:34 Lind-Kovacs, Cora, <cora.l...@utoledo.edu> > wrote: > > I like Stefan’s view outlined below. In many cases, we will indeed refine > those structural parameters to get the best possible match. But when I use > a structural model so that I calculate F(hkl) values, and combine that with > the ability to vary all kinds of parameters - some structure-related, some > instrument/setup related, some microstructure related – I would (and have!) > call that a Rietveld refinement. > > Because if I did not, there would be a lot of fun questions to > contemplate: What if I start doing a Rietveld refinement according to the > more restrictive definition that has been used in this discussion - and > then turn off those structural parameters for one reason or another, maybe > to look into hkl-dependent broadening or such (still constrained by the > structure in contrast to simple “profile fitting” where any peak can have > its completely independent parameters!)? Did I just quit doing a Rietveld > refinement? What if I have a problematic atom, maybe on a split site close > to an inversion center, and I turn that one off? Do I stop doing a Rietveld > refinement when I change that variable to “no”? Does any Rietveld > refinement start out as a “non-Rietveld refinement” when we adjust > background, lattice parameters, approximate peak shapes and phase > fractions, to get to a point where we can actually vary the structural > variables without disastrous consequences? And the moment we touch the > structural variables, in the same software, suddenly it becomes a Rietveld > refinement? Do I need to only turn on ADPs, or is the “threshold” that I > must allow atom positions to refine before I can call it Rietveld? If so, > do I need to turn on all atom positions or only some of them? What if I > define rigid bodies, do I still qualify for a Rietveld refinement? > > Just some fun thoughts here! 😉 > > > > Cora > > > > *From:* rietveld_l-requ...@ill.fr <rietveld_l-requ...@ill.fr> *On Behalf > Of *Stefan Seidlmayer > *Sent:* Tuesday, January 16, 2024 9:51 AM > *To:* rietveld_l@ill.fr > *Subject:* [EXTERNAL] Re: Misconduct > > > > Dear all, > > > > I was following the discussion also with great interest, as terminology is > important to distinguish properly between different items. > > > > To my understanding the Rietveld approach was new because it constrained > the fitting of a peak list generated "from a structure" with the refinement > of the profile of the peaks themselves. > > > > Thus I would have the impression that everytime when we use a > constrainement of peak list which is generated from a structure and do not > refine a list of "individual peaks" it is a Rietveld-type refinement. > > > > A Profile Refinement is/was in my current understanding, when the > peak/reflection position is not constrained by the structure parameters, > but can be refined in an arbitrary way, individual for each peak/reflection. > > From the positions refined in this way, one could then determine cell > parameter etc. But this would require a secondary step. First refine all > found peaks/reflection with a common profile. Then determine the lattice > parameters from the refined peak positions etc. > > This is also troublesome as without prior structure "knowledge" it may and > surely is that certain reflection which in fact are overlapping multiple > reflections are improperly identified as "one" reflection. > > This is very the neatness of the Rietveld approach comes into play, as it > generates even overlapping reflections which then generate the total > profile by constraining them with the profile parameters. > > > > So the main point in differentiation is then in my eyes: > > Rietveld-Refinement: The use of a structure generated peak list which is > constrained with profile parameters. > > > > Profile-Refinement: Profile refinement can work on individual peaks with > NO structure information at all. > > > > Pawley-Refinement then is logically a little bit of a Hybrid, as the > reflection list is pre-generated from the structure. But the intensities > are just matched to best fit the profile. While in a true Rietveld even the > reflection intensities is always generated from the underlying structure > model applied in the Rietveld refinement. > > > > What do you think of this point of view? Am I overlooking something? > > > > Best regards > > > > Stefan Seidlmayer > > > > > > > > Am Di., 16. Jan. 2024 um 15:23 Uhr schrieb Alan W Hewat < > alan.he...@neutronoptics.com>: > > Ha ! When Terry Sabine proposed to call it Rietveld Refinement, I told him > that Rietveld was already "refined". > > > > ________________________________ > Dr Alan Hewat, NeutronOptics > Grenoble, FRANCE (from phone) > alan.he...@neutronoptics.com > +33.476984168 VAT:FR79499450856 > http://NeutronOptics.com/hewat <http://neutronoptics.com/hewat> > _______________________________ > > > > > On Tue, 16 Jan 2024, 15:11 , <alberto.martine...@spin.cnr.it> wrote: > > Dear all, > > I'm following this interesting discussion. It seems to me that sometimes > there is an improper use of terminology, in particular when we talk > about "profile refinement"; in reality, we all know that it is about > "profile fitting". Or did I miss something? > > sorry for the pedantry. > Alberto > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > Please do NOT attach files to the whole list <alan.he...@neutronoptics.com > > > Send commands to <lists...@ill.fr> eg: HELP as the subject with no body > text > The Rietveld_L list archive is on > http://www.mail-archive.com/rietveld_l@ill.fr/ > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > Please do NOT attach files to the whole list <alan.he...@neutronoptics.com > > > Send commands to <lists...@ill.fr> eg: HELP as the subject with no body > text > The Rietveld_L list archive is on > http://www.mail-archive.com/rietveld_l@ill.fr/ > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > Please do NOT attach files to the whole list <alan.he...@neutronoptics.com > > > Send commands to <lists...@ill.fr> eg: HELP as the subject with no body > text > The Rietveld_L list archive is on > http://www.mail-archive.com/rietveld_l@ill.fr/ > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > Please do NOT attach files to the whole list <alan.he...@neutronoptics.com > > > Send commands to <lists...@ill.fr> eg: HELP as the subject with no body > text > The Rietveld_L list archive is on > http://www.mail-archive.com/rietveld_l@ill.fr/ > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > >
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