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

[cockcroft...@gordon.cryst.bbk.ac.uk]

In reply to an earlier email by Lachlan Cranswick:

>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?

Use a Rietveld with mu.r built in that takes account of the
absorption - though to some extent the success of this will depend
on the severity of the absorption. [As an aside, the solution
to this problem at synchrotron X-ray sources is to use
hard X-rays, e.g. as practiced on Andy Fitch's diffractometer
at the ESRF, Grenoble - as a further aside, it would be
interesting to hear the % number of samples collected for
structure refinement/solution using capillaries on this
instrument?]

>(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)

Diluants are certainly useful - diamond powder may be very good if
a lower background is required; othewise amorphous organic materials
("instant coffee" granules, etc. :-) may be better. Resins and
glues can also be used.

>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)

Did the Round Robin distinguish between the two types of
transmission mode - flat and capillary? Certainly the recent
round robin organised by IG-BCA gave an unambiguous result
in favour of capillary: the various back-packed, side-packed,
dusted on grease, etc. flat plate samples all gave very poor
resuls.

>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?

Yes, capillaries do cost money, but not that much compared
to the cost of a diffractometer. And if they give better data
for refinement purposes, then the effort and cost of using them
is surely worthwhile?

Jeremy Karl Cockcroft

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