convergent-beam electron diffraction

Selected-area diffraction shines a wide, parallel beam and gets a pattern of sharp dots. Convergent-beam electron diffraction (CBED) does the opposite: it squeezes the electrons into a narrow cone that comes to a fine point on the specimen, only a few nanometres across, like focusing sunlight through a lens onto a pinhead. Because the beam now arrives from a whole spread of angles at once, each diffraction spot spreads out into a filled disc rather than a point. Those discs are not empty — they are full of fine detail, and that detail is where the extra information lives.

Think of it this way: a parallel beam asks the crystal one question (does this reflection appear?) from one direction. A convergent cone asks the same question from many directions simultaneously, so inside each disc you see a whole 'rocking curve' of how the intensity changes with angle. From this you can read things a spot pattern cannot give. Fine parallel fringes inside a disc (Kossel-Mollenstedt fringes) let you measure the specimen thickness to a few nanometres. Sharp lines from higher layers of the reciprocal lattice (higher-order Laue zone, or HOLZ, lines) fix the three-dimensional cell and even lattice strain. Most powerfully, the symmetry of the whole pattern reveals the crystal's point group and space group from a region only nanometres wide — something no other single technique does so directly.

The special power is telling a centrosymmetric crystal from a non-centrosymmetric one. In ordinary kinematic diffraction, Friedel's law makes the reflection (hkl) and its opposite (-h-k-l) equally bright, hiding any lack of an inversion centre. But electrons scatter so strongly that they diffract many times over inside the crystal (dynamical scattering), and that repeated scattering breaks Friedel's law — so the CBED pattern can show a genuine handedness, exposing the absence of an inversion centre and pinning down the space group. The honest limits: CBED needs a thin, clean, well-oriented crystal held very still; the tiny focused probe contaminates and can damage delicate materials; and interpreting the fine detail usually leans on dynamical simulation, not eyeballing alone.