Precession electron diffraction

Precession electron diffraction (PED) is a key technique available to electron microscopists to elucidate the structure of inorganic crystals. A PED pattern is formed by the summation of a series of diffraction patterns whose incoming beam is tilted with respect to the optic axis. Tilting the beam in this way excites a Laue circle of reflections that will, if the precession angle is sufficiently large, excite many fewer (strong) reflections than if the beam were exactly parallel to the zone axis. In PED, In PED, the precessed beam forms a hollow cone, which corresponds to an annulus in the LACBED disk whose radius is twice the precession angle. The general trend when undertaking PED is to use the largest possible precession angle. The precession method involves tilting the beam away from the optic axis of the probe-forming lens and thus any spherical aberration of this lens causes the probe diameter to increase approximately. For structure solution, the PED intensities are not strictly kinematical in nature but can in many cases be treated like kinematical intensities and be used to solve structures.