Re: [sdpd] size broadening and diffracting volume

[Armel Le Bail <alb...@cristal.org>]

>If this is so, where do you define the physical surface of the sampleholder
>to be, right within the plane that is exactly tangentiel to the focussing 
>circle?

I can define nothing in a few lines. Good definitions need 56 pages
(chapters 7 & 8 in the book "Introduction to X-Ray Powder Diffractometry"
by Ron Jenkins and Robert L Snyder, Wiley, 1996). In this book,
the figure 8.15 shows the magnitude of various errors in peak
position encountered in powder diffractometers. A similar figure
giving the magnitude of various errors in peak broadening (symmetrical
as well as asymmetrical), maybe as a function of sample thickness and
sample absorption, is lacking. By imposing only one standard
(SRM 660a), we are admitting implicitly that no such broadening
error can occur. A too simple view.

>Having said this, I can add to what Lutterotti said: 'Should we have
>different standards for different absorption?' the following: Could we
>just, what I did yesterday, use a very low absorbing good crystalline
>organic compound, spray a very thin layer on a non-absorbive Silicon sample
>holder and use the resulting peak widths as 'the' instrumental widths.

I would say yes. Provided the pattern of the sample from which that
instrumental width will be deconvoluted is recorded in exactly the same
conditions (thickness, absorption, etc). But, when submitting your
manuscript,  be prepared to answer to a reviewer question like :
"Please resubmit after having used SRM660a as a reference
material for the instrumental aberrations".

Yes, I think we should have different standards for different absorption,
when absorption matters on peak broadening, obviously. Effects can
vary a lot according to the diffractometer geometry, the beam (parallel
or focusing), and the radiation (neutron, X-rays). LaB6 is a stupid
standard for neutron diffraction, for instance, unless you select the
adequate isotope.

Best,

Armel


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