On the alignment for precession electron diffraction

Precession electron diffraction has seen a fast increase in its adoption as a technique for solving crystallographic structures as well as an alternative to conventional selected-area and converged-beam diffraction methods. One of the key issues of precession is the pivot point alignment, as a stationary apparent beam does not guarantee a fixed pivot point. A large precession tilt angle, along with pre-field and post-field misalignment, induces shift in the image plane. We point out here that the beam should be aligned to the pre-field optic axis to keep the electron illumination stationary during the rocking process. A practical alignment procedure is suggested with the focus placed on minimizing the beam wandering on the specimen, and is demonstrated for a (110)-oriented silicon single crystal and for a carbide phase (∼20 nm in size) within a cast cobalt–chromium–molybdenum alloy.

► Beam tilt and misalignment lead to image shift for precession electron diffraction.
► The sample image should be stationary with respect to the beam for PED.
► The precession angle is mainly determined by the lower scan.
► PED is successfully used to identify Cr23C6 carbides of ∼20 nm in size.