Re: [sdpd] For Powder Indexing: New Truecell software and Truecell enabled Chekcell update

[L.M.D.Cranswick...@dl.ac.uk]

> I have the reverse problem:  one superstructure reflection forces me to
> quadrupole the three parameters, i.e. a, b and c of an orthorhombic basic
> cell, which multiplies the volume by 64.  However, I suspect there might be
> a smaller unit-cell, with a different orientation of the unit-cell axes,
> and possibly a different crystal system.  I am wondering how to check this
> and find possible smaller cells that would index every peak.  In addition
> to the superstructure reflection, there are many new peaks, weaker,
> however, they are at higher angles where there are too many choices of
> indexation in the 4a x 4bx 4c supercell.  Anybody has an idea how to tackle
> this problem?
> Every suggestion will be welcome.
> Thank you.
>
> Georges
>

In the case of the Crysfire Suite - have you tried using all 7
applicable indexing programs on this data (ito, treor, dicvol,
taup, lzon, fjzn and kohl)?
   http://www.ccp14.ac.uk/tutorial/crys/
(Plugging the resulting Crysfire Summary file into Chekcell
would allow you to quickly evaluate the various solutions which
could be many and varied)

Each indexing program has its various strengths and one can
succeed where all others fail.  Are there trace peaks that 
could be removed to see what resulting cells occur?  The
low precision of the trace impurity peak positions could also
be throwing things out.

-----

Another option is to use fundamental parameters fitting of
the peaks to allow for the geometry abberations of Bragg
Brentano XRDs that cause peak position changes.  As said on
previous occassions, using Fundamental Parameters can 
gives peak results from Bragg Brentano XRDs, as though the
data was collected on an "ideal" XRD.

XFIT peak profiling software for Win95/Win98 can do the job here:
(but you have to know the slit and geometry settings of
the XRD equipment your data was collected with)
Example FP examples at:
  
   http://www.ccp14.ac.uk/tutorial/xfit-95/fun1.htm
   http://www.ccp14.ac.uk/tutorial/xfit-95/liti/rwfunpar.htm
   

I have not got a tutorial on-line comparing the two main peak profiling 
methods for indexing  - but using fundamental parmeters peak profiling 
can make quite a positive difference to the Figure Of Merits and 
suggested cells produced by the indexing programs. 

Lachlan.

> =====================================================================
> 
> At 06:10 AM 5/26/00 +0100, you wrote:
> >
> >There is are new addition to the LMGP suite by Jean Laugier and 
> >Bernard Bochu called "Truecell".
> >
> >Truecell searches for more symmetric, higher symmetry cells that could
> >be a "better" solution to powder indexing data. This can be useful where
> >powder indexing programs favour small, low symmetry cells; and a
> >higher volume, higher symmetry cell could be a better solution.
> >
> >The Chekcell graphical powder indexing helper and spacegroup
> >assignment software now has a version of Truecell included with it - 
> >accessible via the "Cell Parameters" window. 
> >
> >(Truecell (and Chekcell) can read *.SUM Summary results files generated 
> >by the Crysfire Powder Indexing Suite by Robin Shirley)
> >
> >Tutorial runthroughs showing Truecell in action are available
> >from the CCP14 website:
> >
> >  Basic Demonstration of Truecell on Y2O3 Crysfire results
> >    http://www.ccp14.ac.uk/tutorial/lmgp/trucella.htm
> >
> >  Using Truecell on a high FOM monoclinic trial solution
> >    http://www.ccp14.ac.uk/tutorial/lmgp/truecell_large_hex.htm
> >
> >----
> >
> >Truecell and Chekcell Download:
> >  UK: http://www.ccp14.ac.uk/ccp/web-mirrors/lmgp-laugier-bochu/
> >  CA: http://ccp14.sims.nrc.ca/ccp/web-mirrors/lmgp-laugier-bochu/
> >  AU:
> ftp://ftp.minerals.csiro.au/pub/xtallography/ccp14/ccp/web-mirrors/lmgp-laug
> ier-bochu/
> >
> >
> >Crysfire Powder Indexing Suite Download:
> >  UK: http://www.ccp14.ac.uk/ccp/web-mirrors/crys-r-shirley/
> >  CA: http://ccp14.sims.nrc.ca/ccp/web-mirrors/crys-r-shirley/
> >  AU:
> ftp://ftp.minerals.csiro.au/pub/xtallography/ccp14/ccp/web-mirrors/crys-r-sh
> irley/
> >


-- 
Lachlan M. D. Cranswick

Collaborative Computational Project No 14 (CCP14)
    for Single Crystal and Powder Diffraction
Daresbury Laboratory, Warrington, WA4 4AD U.K
Tel: +44-1925-603703  Fax: +44-1925-603124
E-mail: l.cranswick...@dl.ac.uk  Ext: 3703  Room C14
                           http://www.ccp14.ac.uk