This is an archive copy of the IUCr web site dating from 2008. For current content please visit
https://www.iucr.org.
Next: Moving crystal and moving film methods
Up: Laue, Powder and Single Crystal Methods
Previous: The Laue method
Now try a different experimental arrangement. Rather than using white
radiation, take monochromatic X-radiation of one fixed wavelength and
place the crystal in front of the beam. If one plane is set at exactly the
correct value of to reflect, then we observe one and only one reflected
beam from that crystal. Imagine now, still holding the crystal fixed at the
angle , we rotate the crystal around the direction of the incident
X-ray beam so that the plane causing a reflection is still set at the angle
relative to the X-ray beam. The reflected beam will describe a cone
with the crystal at the apex of the cone. Now imagine the situation when we
have not one crystal but we have a hundred crystals each of them set so
that one plane is at exactly the right reflecting angle, to the incoming
beam. We will now have a hundred reflected beams each giving us one
observable point. Imagine now this clump of a hundred crystals is rotated
about the axis of the incident X-ray beam. We will now have a hundred
cones traced out by these reflected beams. Now consider what would
happen if we had a powder of our material which may consist of a
hundred million crystals. If the powdered sample is put into the beam of
X-rays, there will be many crystals in that powder which will be in a
position to reflect the incident beam and there will be enough of them to
get the effect of not point reflections but of a continuous series of point
reflections which will be lying along the arc of the cone that we previously
imagined to exist. This is the basis of the so-called powder or Debye-Scherrer
method which is probably the most common technique used in
X-ray crystallography.
Figure 21
|
The powder camera (Fig. 21) consists of a metal cylinder at the centre
of which is the sample. The powdered material is often glued onto a glass
rod with clear fingernail varnish. A strip of X-ray film is placed inside the
cylinder. Punched into one side of the film is a hole for the beam
collimator and punched into the other side 180 away, is another hole
through which a beam catcher can be placed. The camera is closed by a
light-tight lid and placed in front of the X-ray beam. The pattern on the
film is shown on the right of Fig. 22.
Figure 22
|
This technique can be adapted to photographing wires and sheets of
metal. A flat film is also commonly used for recording reflections at small
angles.
Next: Moving crystal and moving film methods
Up: Laue, Powder and Single Crystal Methods
Previous: The Laue method
Copyright © 1981, 1997 International Union of Crystallography
IUCr Webmaster