The Laue method is mainly used to determine the orientation of large single crystals.
White radiation is reflected from, or transmitted through, a fixed crystal.
The diffracted beams form arrays of spots, that lie on
curves on the film. The Bragg angle is fixed for every set
of planes in the crystal. Each set of planes picks out and
diffracts the particular wavelength from the white radiation
that satisfies the Bragg law for the values of d
and q involved. Each curve therefore
corresponds to a different wavelength. The spots lying on
any one curve are reflections from planes belonging to one
zone.
Laue reflections from planes of the same zone all lie on
the surface of an imaginary cone whose axis is the zone
axis.
Experimental
There are two practical variants of the Laue method, the back-reflection and the
transmission Laue method. You can study these below:
Back-reflection Laue
In the back-reflection method, the film is placed between the x-ray source and
the crystal. The beams which are diffracted in a backward direction are recorded.
One side of the cone of Laue reflections is defined by the transmitted beam. The film
intersects the cone, with the diffraction spots generally lying on an hyperbola. |
|
Transmission Laue
In the transmission Laue method, the film is placed behind the crystal to record
beams which are transmitted through the crystal.
One side of the cone of Laue reflections is defined by the transmitted beam. The film
intersects the cone, with the diffraction spots generally lying on an ellipse. |
|
Crystal orientation is determined from the position of the spots. Each spot can be
indexed, i.e. attributed to a particular plane, using special charts. The Greninger chart
is used for back-reflection patterns and the Leonhardt chart for transmission patterns.
The Laue technique can also be used to assess crystal perfection from the size and
shape of the spots. If the crystal has been bent or twisted in anyway, the spots become
distorted and smeared out.