X- ray diffractometers
X- ray diffractometers: X - ray diffraction is one of the earliest methods for studying the structure of solids. In the process of diffraction, electromagnetic waves of a given frequency but different phases interact to produce constructive interference (bright spots on the film exposed to the light) and destructive interference (dark spots). By a careful analysis of the diffraction patterns, very accurate values of the lattice parameters (unit cell dimensions) can be inferred.
X- ray diffractometers:
When high energy electrons are incident on metallic surfaces they knock off the bound electrons from even the inner shells of the atoms. When electrons from the higher shells undergo a transition into the lower (now partially empty ) shells, X-rays are emitted. E.g., the line in copper ( = 154.18 pm ) is produced by the n = 2 to the n = 1 transition, while the line ( = 139.22pm ) corresponds to the n = 3 n = 1 transition. Suitable filters are used to obtain monochromatic beams of X-rays. The sample to be studied consists of either a powdered form of the solid or a single crystal. The Debye-Sherrer method is for powders. A photographic film is rolled over the sample and the diffraction pattern traces a cone around the incident beam. The crystallites in the powder are arranged in random directions. If the angle of incident light is on any plane, then the diffracted light is at an angle of 2 from the incident beam.
The powdered sample is also rotated in the plane containing the X-rays to give all possible orientations of the crystallites towards the incident beam. Since the powder of each substance has a characteristic diffraction pattern, powder diffraction is extremely useful in identifying materials.
In the single crystal method developed by Bragg, the single crystal is mounted on a platform. The platform (and the mounted crystal) can be rotated in four different circles along four axes so that the diffraction pattern from every plane of the crystal can be observed in the detector. In modern diffractometers most tasks such as rotating the crystal, analyzing the data ( except growing the crystal ! ) are automated and controlled through computer programs. The schematic diagrams of the powder diffractometer and the single crystal diffractometer are shown in Figures 1 (a) and 1 (b) respectively.
Figure 1 (a) Schematic diagram of a Powder diffractometer.
Figure 1 (b) Single crystal diffractometer