[sdpd] Re: Minimum coherent domain size

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

--- In sdpd...@yahoogroups.com, Alberto Martinelli <amartin...@c...> wrote:
> I would like to calculate the theoretical minum size of the coherent
> diffracting domain that can be pointed out by my instrument.
> does anybody know a theoretical formula giving this value?

I do not believe that this minimum coherent domain
size can be established by a complex theoretical
formula. This seems to be a philosophical question
instead ;-).

I would say that any instrument (a powder diffractometer)
is able to point out to the same minimum size, whatever
the wavelength, the resolution (unless there is no resolution
at all), etc.

In fact, the minimum size is necessarily corresponding
to a minimum of multiples of the cell parameters.

If the size is smaller than the cell parameters, then 
you cannot pretend to any three-dimensional order, so that
you will not observe any Bragg peak, and will not be
able to extract any size by formula that are using
the Bragg peaks (their integral breadth, variance, 
Fourier transform, etc).

So, my theoretical formula would be :
         Sm ~ 2P

Sm = Size minimum
P = cell parameter

This formula leads to funny things when the values
of P are quite large. For a cubic protein with P=1000 A,
then the minimum size would be ~2000 A. For a simple
perovskite with P = 3.6 A, then Sm ~ 7.2 A, but
probably this is so small that you would think
your sample being amorphous. But probably if the protein 
is not ordered to more than 2 cells, then the powder
pattern would look like amorphous too, in spite
of this 2000A size...

Hope I was enough unclear ;-).

Best,

Armel

    





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