Dear All,
I also had a similar issue in my PhD thesis when using the that-time
GSAS: wanting to determing d-spacings of unindexed peaks in
half-interpreted diffraction pattern (it was magnetic superstructure
peaks for which I did not have a model).
I may add a small trick to the technique described by Thierry Roisnel,
which is helpful, if you do not access to the individual peaks of the
phase (step 4). You may have a tetragonal cell, e.g. a = b = 0.1 Å and
c = d, with d to be refined and using an appropriate profile function.
If you then put an atom with occupancy 1 at 0 0 0 and one of the same
kind with occupancy -1 at 0 0 0.5, then you get a finite structure
factor for all 00l with odd l and a zero structure factor for all even
l. By this method you have fewer higher order reflections of that
artificial phase, which might interfere with the rest of the pattern.
The first higher order peak is 003, if I get it correct at the moment. I
am not sure whether this worked really like this (it was GSAS at the end
of the 1990s), because some programs do not allow negative occupancy. It
is also possible that I did the thing by considering antiparallel
magnetic moments at the mentioned fractional coordinates.
I, however, definitely enjoy using software where you can combine
Rietveld refinement with individually fitted peaks without relying to
such tricks.
Best regards
Andreas Leineweber
Am 25.04.2022 um 17:36 schrieb Magnus Sørby:
Dear Thierry and Alan,
@Thierry: Thank you very much! This is a great solution.
@Alan: I agree that we should strive to use as few and as meaningful
refinable parameters as reasonably possible, not just in Rietveld
refinements but in all kinds of modelling. The task I’m engaged in now
is to estimate crystallite sizes in Si particles that are produced
along with a lot of non-crystalline stuff in a semi-industrial
process. Taking more time to produce “clean samples” is therefore not
an option (although it would surely be the best advice for a pure
structure determination/refinement study). Since there are a lot of
sample and time is an issue, I don’t have the luxury to spend a lot of
efforts on the other features in each dataset; at least not for the
moment. I’m therefore more than happy to take some shortcuts in the
description of the non-Bragg parts of the data.
Best regards,
Magnus
*From:*rietveld_l-requ...@ill.fr <rietveld_l-requ...@ill.fr> *On
Behalf Of *Alan W Hewat
*Sent:* 25 April 2022 15:50
*To:* rietveld_l@ill.fr
*Subject:* Re: Introduction of non-structural peaks in Fullprof
Dear Thierry and Magnus.
Introducing an extra pseudo-phase, and then editing out all the peaks
you don't want, is ingenious, but is it really less work than creating
a background file as Rietveld advocated?
The whole idea of Rietveld was to refine only physically meaningful
parameters. The more parameters you introduce, the less confidence you
can have in any of them being physically meaningful, especially if you
then have to constrain some of them.
Alan
________________________________
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 Mon, 25 Apr 2022, 14:53 Thierry Roisnel,
<thierry.rois...@univ-rennes1.fr> wrote:
Dear Magnus,
Here is a way to fit independent peaks in FullProf :
1. edit .pcr file and add for every independent peak an artificial
phase to treat in Profile Matching refinement mode (Jbt=2 and
Irf=0). You can use P m m m space group for example and adjust the
a parameter to fit with the diffraction peak 2theta position, b
and c parameters can have arbitrary values, smaller than a. So,
the special reflection will have 1 0 0 indices.
2. add refinement of one special reflection (Nsp_Ref = 1)
3. Run FullProf and update .pcr file, to provide Irf = 2
4. edit the created xxn.hkl file ( n for phase #n) and remove all
reflections excepted the first one (1 0 0)
5. edit .pcr file and refine peak position and profile parameters
for the special reflection you want to fit.
The .pcr file will look as follows :
Phase #2 : fit a single peak
!
!Nat Dis Ang Pr1 Pr2 Pr3 *Jbt Irf* Isy Str Furth ATZ Nvk
Npr More
0 0 0 0.0 0.0 1.0 *2 2* 0 0 0 32516.641 0 7 1
!
!Jvi Jdi Hel Sol Mom Ter Brind RMua RMub RMuc Jtyp
*Nsp_Ref* Ph_Shift N_Domains
0 0 0 0 0 0 1.0000 0.0000 0.0000 0.0000 0
1 0 0
!
!
*P m m m* <--Space group symbol
!-------> Profile Parameters for Pattern # 1 ----> Phase # 2
! Scale Shape1 Bov Str1 Str2 Str3
Strain-Model
0.1252800E-04 0.00000 0.00000 0.00000 0.00000 0.00000 0
0.00000 0.000 0.000 0.000 0.000 0.000
! U V W X Y GauSiz LorSiz
Size-Model
0.005115 -0.007545 0.020850 0.012780 0.020016 0.000000
0.000000 0
0.000 0.000 0.000 0.000 0.000 0.000
0.000
! a b c alpha beta gamma
#Cell Info
*3.400000 0.500000 0.500000 90.000000 90.000000 90.000000**
0.00000 0.00000 0.00000 0.00000 0.00000 0.00000*
! Pref1 Pref2 Asy1 Asy2 Asy3 Asy4 S_L D_L
0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.01849
0.02007
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
*! Special reflections:**
! h k l nvk D-HG^2 Cod_D-HG^2 D-HL Cod_D-HL
Shift Cod_Shift Del_U/sig1 Code Del_Y/gam2 Code
1 0 0 0 0.0000 0.000 0.00000 0.000
0.00000 0.000 0.00000 0.000 0.00000 0.000*
I hope you will success in what you want to do.
Regards
Thierry Roisnel
Le 25/04/2022 à 11:18, Magnus Sørby a écrit :
Hi,
I’m playing with Fullprof again for the first time in many
years since I’ve changed employer and lost my access to Topas.
Is it possible to introduce independent peaks (i.e. peaks
whose positions are not determined by a unit cell) with
refinable positions, intensities and shapes in Fullprof? I’ve
found this possibility very useful in Topas for dealing with
features in the datasets that are not of interest and not
possible to possible to describe with a crystalline phase e.g.
diffuse scattering from an amorphous component.
I’m aware of the option to include a self-defined background
(.bac file) with refinable scale, but this is time-consuming
workaround.
Best regards,
Magnus
----
Magnus H. Sørby, PhD
Senior Scientist - Materials Science and Analytical Chemistry
Cenate AS
Norway
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