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

[Armel Le Bail <armel...@fluo.univ-lemans.fr>]

>I should also point out that what I meant by higher angles
>might more strictly be-calleded intermediate angles, i.e. those where
>the alpha1,2 peak splitting is present but not resolved. Some of the
>peaks at these sort of angles may be required for indexing and the human
>eye can be very good (with practice) at distinguishing single from multiple
>peaks, but less so when two wavelengths are present and all the peaks are
>multiple by definition.
>
>So I beg to differ with you Armel on this point ....

Well, even if not completely resolved, the alpha-2 component is
removable by one of the 50 or so alpha-2 correction methods 
(almost all published in the Journal of Applied Crystallography,
and all pretending to be the best). This is not a job for the eyes.
Moreover, when the FWHM is as low as 0.04°(2-theta) in the
5-50°(2-theta) range, the alpha-2 component is detectable at angle
as low as 20°, even by old eyes like mines. The old good Rachinger
method removes Kalpha-2 almost perfectly, at any angle, so that you
may obtain the peak positions exactly like if you had a primary beam
monochromator. A Round Robin about the precision of the extracted
peak positions either from Kalpha or Kalpha-1 should not conclude
clearly in favour of the Kalpha-1 option, in my opinion, but you
may organize one.

I don't see how your arguments could convince me. Our disagreement
will continue for ever.

Best,

Armel Le Bail
http://www.cristal.org/course/



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