Re: [sdpd] how to purchase a high resolution diffractometer

["L. Cranswick" <L.M.D.Cranswick...@dl.ac.uk>]

If transmission setup is recommended for Rietveld
as given below:
What methods are used for handling absorption/packing
density so that thermal parameters can still make some
sense as well as doing occupancy work?

(Ian Madsen at CSIRO was using industrial diamond
as a dilutant for the capillaries which was working
quite well - though reflection seemed to be much
nicer negating problems due to absorption with 
transmission mode data)

I would have thought for "generic" inorganics - reflection 
mode would be preferable for Rietveld structure
refinement(?) - assuming preferrered orientation is not that
big a problem.  Am away from the office -  thus from possibly
flawed memory - transmission mode datasets on the 2nd 
Rietveld Refinement Round Robin paper (Hill and Cranswick) 
for laboratory sources gave some of the worst results?  
(though this may not be a statistically valid given
the limited number of lab transmission mode datasets
that were collected) 

Routine loading of capillaries can also be tedious and 
an expensive consumable after a while.  
I take it that with the original request - minimizing 
running and consumable costs are important?

Lachlan.

PS:  What are good sources of XRD capillaries these days
and what is the pricing?

PPS:  Also, for indexing purposes - how is the systematic
non-linear peak offset due to absorption in the capillary 
handled?  Also, I believe most Rietveld codes do not include 
this offset as an option when dealing with capillary data?

> I basically agree with Robin Shirley's earlier comments. I would add that 
> the transmission setup can be optimized further than we do here regarding 
> resolution - our Bragg-Brentano setup has narrower peak widths than the 
> equivalent capillary transmission setup, but that is our choice! :-) 
> As usual there is a compromise between resolution and intensity, and our setup
> is more for Rietveld refinement than say indexing. 
> 
> Having a choice of two diffractometers, it is clear that certain types of
> experiment are done better on one than the other and vice versa. Routine
> sample identification if usually better on the reflection setup, whereas
> Rietveld work is nearly always better with the transmission geometry. Also
> capillaries are easier to cool - routine with a liq. N2 cryostream, somewhat 
> less routine with a liq. He cryostat. This is something you should consider 
> when working with organic materials where the high-angle part of the
> pattern may be wiped out by thermal motion at room temperature.
> 
> Jeremy Karl Cockcroft,
> School of Crystallography,
> Birkbeck College, University of London, 
> United Kingdom.


-- 
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


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