Wydarzenia w AGH

Wykład zatytułowany Application of electron beam precession methods for structure and short-range order analysis in nanoscale materials research wygłosi dr Maciej Zubko, prof. UŚ (Uniwersytet Śląski, Wydział Nauk Ścisłych i Technicznych).

Miejsce: ACMiN AGH (ul. Kawiory 30, bud. D-16, sala audytoryjna 1.02 A)

Streszczenie

Classical analysis of the crystal structure is based on describing the material by providing information regarding the lattice parameters, space group, crystal symmetry and the positions of atoms within a unit cell. Such an approach is widely and commonly used as a tool for the analysis of crystalline bodies. Well-established mathematical formalism and computational procedures allow for a flawless crystal structure solution of the studied materials based on X-ray or neutron radiation diffraction. Currently, also methods that employ electron radiation are rapidly developing. The inability to obtain good quality single crystals of at least a few tens of micrometres in size usually prevents the use of X-ray diffraction to determine the crystal structure of unknown materials. In such cases, electron diffraction has several advantages, including that it can be used to study nanometre-sized objects. In order to solve the crystal structure of the studied material, it is sufficient to measure the Bragg reflections intensities because the periodicity of the crystal lattice implies the formation of a discrete diffraction scattering distribution in the reciprocal space. However, any type of disorder in the real crystal structures causes additional diffuse scattering observed beyond the Bragg reflections. For certain types of compounds, the occurrence of diffuse scattering is quite a common phenomenon, and its analysis is necessary for a proper understanding of the crystal structure. Since an electron beam interacts with matter approximately 102 ÷ 103 times stronger than in the case of X-rays, therefore it is more suitable to observe subtle diffraction effects.

The Precession Electron Diffraction (PED) method is a relatively new technique developed to solve nano-object crystal structures. Applying the PED method allows for a significant reduction of dynamical diffraction effects, which makes the diffuse scattering analysis much more reliable. In the PED technique, an electron beam deflected at a specific angle undergoes precession along the envelope of the cone, and then, after passing through the sample, the diffraction beams are deflected back towards the optical axis of the microscope. Measurements are made by rotating the sample around an arbitrary crystallographic axis with a preset step and recording successive diffraction images. Additionally, the other advantage of the PED is the possibility to record 3D diffraction data from the large segment of the reciprocal space (it is sometimes called reciprocal space tomography), which provides detailed information about the distribution of the diffuse scattering in the reciprocal space.