5.3. Scattering and diffraction (Optical diffraction diagrams)
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Optical diffraction diagrams
In the book Atlas of Optical Transforms
(G. Harburn, CA Taylor, TR Welberry, Ed G. Bell & Sons, London
1975) several optical analogues are shown which can help in order to
interpret Xray diffraction patterns. Of the nearly 400
examples provided, we have selected a few which can help illustrate what has been shown in this chapter.
In
each figure, the diagrams on the top row represent the system where the
light is diffracted, and the lower row shows the diffraction
produced. The exception is the final figure, which shows the diffraction
patterns at the top and the objects obtained from the diffraction at
the bottom.

We
present here the simplest object (a circle) and the combination of two
simple circles, showing the effect of the distance (spacing) between
them. As this spacing increases, the number the diffraction fringes
also increases and they appear much closer (that is the
"reciprocal" effect. See the reciprocal lattice). 

When the object is combined into lines, the corresponding diffraction fringes occur perpendicular to the original line.
If the object forms a 2dimensional lattice (figure on
the right), the diffraction pattern produces another lattice,
reciprocal of the original. The variations in intensity in the latter
are due to the finite size of the 2dimensional object.


The
original object is getting slightly complicated. It can be seen as
an idealized representation of several chemical molecules: benzene,
toluene and nitrobenzene. 

The same molecule can show polymorphic structures, ie different crystal structures.
The diffraction diagrams apparently show different
distributions of intensities, but in both cases one can discover how
the diffraction pattern reveals (somehow) the diffracting object, in
this case a molecule of benzene.


Distortions
of the periodicity in the direct lattice (figures at the top) are
transformed into the diffraction patterns as blurred lines. 

If
the crystal is composed of a set of discontinuous mosaics (left), the
diffraction maxima on the diagrams become wider and diffuse.
When the mosaics also change their orientation, the diffraction
diagrams show emergent circles. Taken to the limit, this would result in
a complete circle diagram, typical of microcrystalline powder.


If
the sample contains two or more orientations in the
lattice (twins), the maxima on the diffraction pattern split, and
if the size of the twin components are small, the split maxima can
appear as lines. 

The
two figures in the lower row show the corresponding images of a
molecule (rhodium  phthalocyanine) as obtained from their diffraction
patterns (upper row). The molecular image is more or less recognizable depending
on the amount of information contained in the corresponding diffraction
patterns.
The figure on the right side shows the same projection of the electron density of this molecule.
