ED and XRD,
are caused by constructive interference of scattered waves, and the
same fundamental laws (e.g., Bragg law,
extinction rules) can be applied for the interpretation of the resulting
diffraction patterns. In both cases, diffraction patterns of powders
and of single crystals appear.
ED and XRD show some distinct differences:
The wavelength of electrons (e.g., 1.97 pm for 300 keV electrons)
is much shorter than that of X-rays (about 100 pm). Therefore, the radius of the Ewald sphere
is much larger and more reflections are observed by ED than by XRD.
The diffraction angles are very small in ED: 0 < Θ
< 2° (cf., XRD: 0 < Θ
scattered by the positive potential inside the electron
cloud (Coulomb interaction), while X-rays interact with the electron cloud. As
the result, the interaction of electrons with matter is
much stronger (106-107×) than that
has the advantage that the diffracted electron beams have a high
intensity and exposure times are in the order of a few seconds.
can directly be observed on the viewing screen of the electron
microscope. Thus, orienting a crystal along a direction can be
by tilting while observing changes of the ED pattern simultaneously.
Furthermore, diffraction patterns can be obtained from very small
crystals selected with a diffracted aperture (Selected Area Electron
Diffraction SAED) and by a focused electron
beam even from nm-sized regions (Convergent Beam Electron Diffraction
The disadvantage of the strong interaction is that multiple
scattering plays an important role, and the intensities of the
reflections are much
by this dynamical
effect. This makes structure determination from ED more difficult
and less reliable than that from XRD data. A method for getting more reliable quantitative data is precession diffraction.