Energy Dispersive Diffraction

Instead of fixing the wavelength and recording diffracted intensity from a crystalline sample over a wide range of angles, in this technique the diffraction angle (2-theta) is fixed and white (or polychromatic) radiation is used. The wavelength (or energy) of the X-rays contributing to any given peak is related to a d-spacing in the sample that generated it and to the 2-theta angle of the detector system collecting it. The diffracted intensities are recorded using an energy discriminating detector in conjunction with multi-channel analyser electronics which sorts the data into bins according to their energy. With this technique all data are recorded simultaneously minimising data collection time and making the technique ideally suited to the real time study of chemical reactions and phase transitions using time slices of < 1 minute.

Since the energy of a diffraction peak from a particular d-spacing can be altered by varying the 2-theta angle of the detector it is possible to place a characteristic peak from a studied reaction at an energy that is optimal for the liquid nitrogen cooled solid state detectors that are used for this work. Typically, high energy radiation having high penetration is used in these experiments so that samples can be contained in relatively thick walled vessels. Also, because of the low 2-theta angles involved, it is convenient to use this technique with heaters and pressure vessels that allow only small entrance and exit holes. By using a 3-element detector system (each separated by about 3 degrees in 2-theta) a wide continuous range of d-spacings can be studied simultaneously.

station 16.4