Hi Carlos
Well, diffraction is a consequence of the crystal lattice repeat (or it
could be a 1-D repeat in a fibre), for without the lattice there could
be no diffraction, only scattering. But the lattice repeat is obviously
directly related to the average distance between atoms and the number of
atoms in the cell, the more atoms there are on average the bigger the
cell, and the number of orders of diffraction you observe, and hence the
number of reflections, is related to the lattice repeat and the
wavelength. To get sufficient independent data for structure solution &
refinement you need rather more unique reflections than atoms, so doing
the detailed calculation you find that the lattice repeat (or cell
volume), the number of atoms and the number of reflections cancel each
other out, and you're left with the requirement that the wavelength must
be equal to the average interatomic distance, plus or minus a factor of
~ 1.5.
-- Ian
> -----Original Message-----
> From: CCP4 bulletin board [mailto:[log in to unmask]] On
> Behalf Of Carlos Frazao
> Sent: 24 January 2007 09:09
> To: [log in to unmask]
> Subject: [ccp4bb] relation between wavelength and
> inter-atomic distances
>
> Hi,
> I have once heard and recently read that "the diffraction
> event results
> from the fact that both the X-rays wavelength and the atomic
> distances
> are of the same magnitude". Although such a relation seems
> appealing I
> am unsure if this is not a mere coincidence. Could someone clarify or
> lead me to a relevant reading.
> Cheers,
> Carlos
>
> --
> **************************************
> Dr. Carlos Frazao
> Crystallography Department
> ITQB-UNL, Av Republica, Apartado 127
> 2781-901 Oeiras, Portugal
>
> Phone: (351)-214469666
> FAX: (351)-214433644
> e-mail: [log in to unmask]
> www.itqb.unl.pt
>
>
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