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All posts by rowlesmr (248)

topic: Phi input for PO_Spherical_Harmonics?  in the forum: Forums topas
rowlesmr #1
Member since Oct 2011 · 248 posts
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Nice.

I also talk about it a little here [1]. I'll need to read the techref some more.



[1] Rowles, Matthew Ryan, and C. E. Buckley. 2017. "Aberration Corrections for Non-Bragg-Brentano Diffraction Geometries." Journal of Applied Crystallography 50 (1): 240-251. https://doi.org/10.1107/S1600576717000085.
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Matthew
topic: Phi input for PO_Spherical_Harmonics?  in the forum: Forums topas
rowlesmr #2
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In reply to post ID 2319
Hi Matias

The refinable Cij prms are scaling factors for the Yij equations as outlined in [1].

The simplest way would be to have one dataset per azimuthal angle, and have independent Cij prms. The next layer of complexity would be to write a parametric equation to describe how each Cij changes with azimuth

Matthew


(Question: what is the alpha thing you're talking about?)




[1] Järvinen, M. 1993. "Application of Symmetrized Harmonics Expansion to Correction of the Preferred Orientation Effect." Journal of Applied Crystallography 26 (4): 525-531. https://doi.org/10.1107/S0021889893001219.
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Matthew
topic: Refinement parameters extraction error  in the forum: Forums topas
rowlesmr #3
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In reply to post ID 2317
Use local parameters

xdd
   ...
   str
      local csL 203
      ...
   str
      local csL 157
      ...

   for strs {
      Out(csL, " %11.5f"\n) 'or whatever the code is that you need.
   }
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Matthew
topic: Qmin for PDF refinement  in the forum: Forums topas
rowlesmr #4
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In reply to post ID 2305
A quick look at Egami and Billinge (p. 170) seems to say that you should extrapolate from Qmin to Q=0, rather than setting the data to zero. They cite Thijsse (1984) were the first ~40 data points of Q[S(Q) - 1] should be fitted with aQ + bQ^3 and use that to extrapolate to zero.

Thijsse (1984) J Appl Cryst 17 61
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Matthew
topic: Qmin for PDF refinement  in the forum: Forums topas
rowlesmr #5
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In reply to post ID 2303
Having never done a PDF refinement...

Do you mean start the model at a point after the beginning of the data? Would start_X be what you're after?

Matthew
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Matthew
topic: FCC stacking faults and PO  in the forum: Forums topas
rowlesmr #6
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In reply to post ID 2301
Yep. 2x2x2 P1 cell gave a few more than 107 structures.

No structures more symmetric than monoclinic had an axis parallel to the cubic [112].

It looks like C2/m is the symmetry of choice, just with all angles == 90.
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Matthew
topic: FCC stacking faults and PO  in the forum: Forums topas
rowlesmr #7
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In reply to post ID 2300
There wasn't a file attached?

I put in the Im-3m structure as the parent, and the P1 str as the distorted one, and got 107 subgroups, so I assume i did the same as you.

I chose blah blag [1,1,2] as the transformation matrix.

.

Looking through the subgroups, the only ones which have a (1,1,2) basis for an axis are monoclinic or triclinic.

I'll make up bigger versions of my P1 cell and see what happens.
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Matthew
This post was edited 2 times, last on 2020-11-15, 05:17 by rowlesmr.
topic: FCC stacking faults and PO  in the forum: Forums topas
rowlesmr #8
Member since Oct 2011 · 248 posts
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In reply to post ID 2298
Now I'm getting into the dim dark recesses of crystallography.

Converting BCC (Im-3m) to orthorhombic


The twinning plane in BCC is (112), so following [1] (cited by [2]), (and fiddling with the order of axes to maintain right handedness), the orthorhombic basis vectors are defined, in terms of the BCC vectors (2.880 Å), as:

A = a - b                        (4.073 Å)
B = (1/2) (a + b - c)       (2.494 Å)
C = a + b + 2c               (7.054 Å)

this gives the c-axis in the faulting direction. This is also triples the volume, so I expect 6 atoms in the unit cell.

I've then manually picked out the atom positions in the new unit cell as
 0     0   0
1/2   0  1/2
 0   1/3 1/3
1/2 1/3 5/6
 0   2/3 2/3
1/2 2/3 1/6

(and it is 6)

By observation, I think it is a B base-centred orthorhombic cell, but I can't get a space group. If I plug it into Topas in P1, the calculated pattern matches BCC, so I'm good there. Platon wants to bring it back to P-3m1. FINDSYM goes back to Im-3m.*

Looking through International Tables, I can't find a SG with the right symmetry. What I think I need is:

B base-centred orthorhombic cell
(0,0,0)+ and (1/2,0,1/2)+
Multiplicity Coords
      2          0,0,0   (or equivalent, eg 0,0,z or x,y,z)
      2          0,y,y    (or equivalent, eg 0,u,z or x,y,z)


Is there such a beast?


*Is there any way to limit the space groups it searches?

[1] Hirsch and Otte (1957) Acta Cryst. 10: 447
[2] Warren "X-Ray Diffraction", p. 305
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Matthew
This post was edited on 2020-11-13, 03:46 by rowlesmr.
topic: FCC stacking faults and PO  in the forum: Forums topas
rowlesmr #9
Member since Oct 2011 · 248 posts
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In reply to post ID 2297
Thanks John

Reading up on how stacking faults are done, that's what I thought. Just need to wrap my head around the nomenclature for describing faulting, and how I can combine stacking faults with twinning.

I'll contact Alan re the SH.
--
Matthew
topic: Modelling peak shifts in fcc structures with stacking faults?  in the forum: Forums topas
rowlesmr #10
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In reply to post ID 2259
This is now of interest to me, but combined with some spherical harmonics PO, for some 3d printed steels.

Interestingly, if you google "diffax peak shift", this thread is my number 1 hit.
--
Matthew
topic: FCC stacking faults and PO  in the forum: Forums topas
rowlesmr #11
Member since Oct 2011 · 248 posts
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In reply to post ID 2294
Answer for 1:

By Inspection of the refined coefficients:
c63m = -c66p;
c43m = 2 c66p;

c60 = -0.5 c40;
c20 = -4 c40;

How can I check the theory for this?
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Matthew
This post was edited on 2020-11-09, 07:19 by rowlesmr.
topic: FCC stacking faults and PO  in the forum: Forums topas
rowlesmr #12
Member since Oct 2011 · 248 posts
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Subject: FCC stacking faults and PO
I've got some printed steels (FCC, austenite) which look like there are stacking faults involved; the 200 peak is too low compared to the 111.

But I've also got some PO involved, which I can (only) model with 6th order SH (two parameters; c41, c61).

Looking at the stacking faults tutorial, Cu (FCC) is done in hexagonal coordinates in order to make the stacking faults align with the c axis. I can do the same with the steel (Fm-3m --> R-3m:H), and I've already done the calculations of the correct unit cell prms and atom positions.


Questions:
How do I constrain the 6 x HCP SH prms (6th order; c40, c43-, c60, c63-, c66+) to have two unique prms as per the FCC unit cell?

OR

How do I make stacking faults work when not parallel to a unit cell axis?

Thanks

Matthew
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Matthew
This post was edited 2 times, last on 2020-11-09, 07:01 by rowlesmr.
topic: Influence of TRIO and TWIN optics selection on Rietveld refinement results  in the forum: Forums topas
rowlesmr #13
Member since Oct 2011 · 248 posts
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In reply to post ID 2292
You should use whatever optics are in the beam path.
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Matthew
topic: Correlation matrix with bootstrapped errors  in the forum: Forums topas
rowlesmr #14
Member since Oct 2011 · 248 posts
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In reply to post ID 2286
Not answering your question, but how long does it freeze for?

I've accidently had do_errors defined when doing some fairly hairy stuff, and while it looks like it's frozen, it is working away in the background. Just as a test, maybe setup a refinement to finish on a Friday afternoon, and give it the weekend to do the errors?

.

Also, your suggestion of local_with_error is a really good idea. I'm wondering if you could do a work around by defining a bunch of prm_with_errors wth unique names, and then have local nameOfChoice = prmWithErrorName;   I don't know if it would work, but it would be worth investigating.

Matthew
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Matthew
topic: polyhedral volume  in the forum: Forums topas
rowlesmr #15
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In reply to post ID 2279
I started writing out the equations, then I gave it up as a bad idea.

I couldn't find any way to easily write general functions (ie prm d12 = Calc_Distance(s1, s2);) in order to calc the volume of a general polyhedron.

The only way I found would be to input series of tetrahedra which make up the polyhedron of choice and calc the volume as a sum.

I know the formula, but lack a simple way to just get an atom-atom distance out in a single line.

Matthew
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Matthew
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