Fyzikální ústav Akademie věd ČR

Head: Lukáš Palatinus

Precession electron diffraction is a new method that allows for the determination of crystal structure of microcrystals from electron diffraction data. It is different from the standard electron diffraction in that the beam is not stationary, but precesses along a cone, and this precessing motion has the effect of partially suppressing the dynamical effects in diffracted intensities. The main advantage of electron diffraction over the x-ray diffraction is the possibility to analyze very small samples, which are inaccessible by x-ray diffraction. On the other hand, the interaction of electrons with atoms in the crystal is more complicated than in case of x-rays, and that makes the structure analysis from electron diffraction data more computationally involved, and, at least for now, less accurate than x-ray structure analysis.

It is very advantageous to combine electron diffraction with x-ray powder diffraction. It is very difficult to solve complicated crystal structures from powder diffraction data, if we do not have an initial guess of the structure. Such a guess can be provided by electron diffraction experiment performed on one microcrystallite from the powder sample. The combination of electron diffraction with single-crystal x-ray diffraction is less frequent, but it has proven to be very useful for multiphase single crystals. The long-term goal of the Department of structure analysis is constructing an experimental and computational base that would combine all these three basic diffraction methods.

The heart of the laboratory of precession electron diffraction is the transmission electron microscope Philips CM120. This microscope allows for a large sample tilt up to ±60°, which is essential for the investigation of three-dimensional structures. The microscope is equipped with the instrument Spinning Star produced by the company NanoMegas, which is responsible for the precessing motion of the beam. The whole setup is completed by the wide-view CCD camera Veleta, and electrometer Pleiades, which measures the intensities of electron beams on the principle of the Faraday cage, and exhibits an unprecedented dynamical range of 24 bits.

The application of precession electron diffraction brings a number of theoretical and technical problems, which are dealt with within the project Electron diffractometer (see Research topics of the Department of structure analysis).

Transmission electron microscope CM120 with accessories for electron precession diffraction. (1) Area detector Veleta, (2) Elektrometer Pleiades, (3) Precession equipment Spinning Star