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Crystal Density
Crystal Density error in TOPAS v.3
Vlad #1
Member since Apr 2018 · 7 posts
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Subject: Crystal Density
At Rietveld refinement (using TOPAS v.3.0) of XRD pattern obtained from Ruthenium-containing material a strange result was obtained-in TOPAS report the value of Crystal density of Ruthenium was given as 37.933(36) g/cm3. Can this error be eliminated? Does it affect of quantitative analysis result?
Structure 1
   Phase name                                Ruthenium
   R-Bragg                                   1.336
   Spacegroup                                P63/mmc
   Cell Mass                                 606.423
   Cell Volume (Å^3)                         26.764(47)
   Wt% - Rietveld                            35.9(20)
   Crystallite Size
      k:  0.89  LVol-FWHM (nm)               6.37(19)
   Crystal Linear Absorption Coeff. (1/cm)   6644(12)
   Crystal Density (g/cm^3)                 37.625(65)
   Lattice parameters
      a (Å)                                  2.6896(21)
      c (Å)                                  4.2722(35)

Site  Np    x           y           z         Atom Occ       Beq
Ru1   6   0.33330     0.66670     0.25000     Ru   1         0.50(21)
johnsoevans (Administrator) #2
User title: John Evans
Member since Aug 2009 · 219 posts
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You need to make your fractional coordinates for Ru1 exactly 1/3 and 2/3 instead of 0.3330 and 0.66670.  Use =1/3; and =2/3;.  The atom will then be on the multiplicity 2 site you expect and the density will come down.  The clue is in the "Np 6" entry in your post.
Vlad #3
Member since Apr 2018 · 7 posts
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Thanks,John. Unfortunately my TOPAS v.3 does not accept atomic coordinates as fractions. Although the density in report is indicated correctly, the Rietveld refinement is not performed correctly. Probably this is a bug in software.
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johnsoevans (Administrator) #4
User title: John Evans
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v3 is a long time ago, but this should work.  Did you type "=1/3;" into the relevant box (if you're using the gui) or into the site line (if you're using an input file. 

What you show isn't a "bug".  You've told topas in this example that x=1 and y=2 so the refinement is correct under that information.
Vlad #5
Member since Apr 2018 · 7 posts
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I use GUI and I type 1/3 and 2/3. But 1/3 and 2/3 turn into 1 and 2. The same thing happens when I use .inp and .pro files.
johnsoevans (Administrator) #6
User title: John Evans
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As in the previous post, type "=1/3;" not just "1/3".  The = and ; indicates a topas equation.
Vlad #7
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Thank you very much for the help. In GUI mode, this did not work, but after editing the .pro file, it was wonderful!!!
rowlesmr #8
Member since Oct 2011 · 199 posts
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In GUI mode, you need to write in "=1/3;" on the codes page, not the values page.

.

And yes, it will affect quant results, as the cell mass will be wrong, and the intensities will be wrong which will bias the scale factors.
--
Matthew
Vlad #9
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It all worked. Thanks again!
This post was edited on 2019-07-07, 05:43 by Vlad.
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