On 4 August 2015 at 11:54, Kotaro SAITO <kotaro.sa...@kek.jp> wrote:
> Or do I miss some basic points about diffraction?
I won't try to address your specific material... and I'm being called to
lunch :-) But for beginners who may be lost in these technical papers, I
will attempt the following trivial explanation
If you have a layered material where two layers A and B are slightly
different you will have super-structure reflections. These will be as sharp
as the main reflections (from the average structure) if the order of the
layers is perfectly regular ABABABAB...
But if the layers only have short-range order eg ABABBABAAB... then these
superlattice reflections will be broadened, and even completely washed out
if the order between layers is completely random. Otherwise the width
delta-d of the superstructure reflections will give you the short range
order length - the shorter the correlation length the broader the
superlattice reflections.
Obviously delta-d doesn't depend on the d-spacing between layers, only on
the length of their order. So the broadening is constant in d-space as
usually plotted for TOF neutron diffraction.
For angular dispersion eg with a constant x-ray or neutron wavelength,
Bragg's law 2d.sin(theta)=lambda comes in. If you differentiate Bragg's law
you will find a simple relation between delta-d and delta-2theta, the line
broadening for angular dispersion measurements.
Alan.
(Everything should be as simple as possible... but no simpler.)
BTW, thanks for using dropbox instead of an attachment. That's the way to
go...
--
______________________________________________
* Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE *
<alan.he...@neutronoptics.com> +33.476.98.41.68
http://www.NeutronOptics.com/hewat
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