Electron diffraction is a technique used to examine solids by firing a beam of electrons at a sample and observing their deflection. The electrons are deflected not as particles but as waves, as in classical diffraction (see Wave-particle duality).
The technique is only used on crystal samples that have a regularly spaced atomic lattice. Most electron diffraction is performed with high energy electrons whose wavelengths are orders of magnitude smaller than the interplanar spacings in most crystals. For example, for 100 keV electrons λ < 3.7 x 10-12 m. Typical lattice parameters for crystals are around 0.3 nm.
The electrons are scattered by interaction with the positively charged atomic nuclei. Electrons are charged, light particles which interact very strongly with solids so their penetration is very limited. Low-energy Electron Diffraction (LEED) and Reflection High-Energy Electron Diffraction (RHEED) are therefore considered to be surface science techniques, while transmission electron diffraction is limited to specimens less than 1mm thick. Transmission electron diffraction is usually carried out in a Transmission Electron Microscope (TEM).
The technique is somewhat similar to Neutron diffraction.