Electron Crystallography :

Novel Approaches to Structure Determination of Nanosized Materials

the 36th international crystallographic course organized at Erice
June 9 to 20, 2004 (dates changed on Dec 2, 2002 so that the close is on Sunday)

Directors : Janos LABAR, Budapest, labar@mfa.kfki.hu; Thomas WEIRICH, Aachen, weirich@gfe.rwth-aachen.de; Xiaodong ZOU, Stockholm, zou@struc.su.se

a NATO Advanced Study Institute,
sponsored by the International Union of Crystallography

Since their inception, crystallographic meetings in Erice have striven to explore frontier topics. News on earlier and future activity can be found in history, summary and statistics.

1. JUSTIFICATION.

From the beginning when it was realised that electrons - like X-rays - are diffracted by the atoms in crystals it was clear that electron diffraction could - in principle - be used in the same way as X-ray diffraction for the determination of crystal structures. However, with increasing understanding of the electron scattering process, in particular by development of n-beam scattering theory during the 50ies of the past century, it became general knowledge that it could normally not be assumed that the kinematical or single-scattering approximation gives even a rough indication of the diffracted intensities. For this reason nearly all activities in this direction were stopped within a few years from that time on. Whilst X-ray diffraction turned over the years into a powerful quasi-automatic method for structure determination, structure analysis by electron diffraction was not seriously developed further over a period of more than twenty years. So it took until 1976 to perform the first structure determination by direct phasing methods from electron diffraction data of an organic compound and another eight years for the first ab-initio structure determination of a heavy-metal oxide from high-resolution electron microscopy images. However, despite these achievements the earlier held skepticism against structural results from electron data are still a common mindset, which might explains why only a few groups around the world actively use electron crystallography for structure determination. On the contrary the increased interest in nanosized materials in research and application calls for reliable methods to analyse atomic structure of extremely small samples which is beyond the capabilities of X-ray diffraction even on a modern synchrotron. Due to the strong interaction of electrons with matter structure determination by electron crystallography is the method of choice to reach this goal.

During the last decade a lot of exciting developments have been made in electron crystallography. They include structural and charge density studies on organic molecules and protein structures, complicated inorganic and metallic materials in the amorphous, nano-, meso- and quasi-crystalline state and also development of new software, tailor-made for the special needs of electron crystallography. Moreover, these developments are accompanied by an upcoming new generation of computer controlled electron microscopes equipped with high-brightness field-emission guns, cryo-specimen holders, ultra-fast CCD cameras and correctors for electron optical distortions. Thus a fast and semi-automatic data acquisition from small sample areas, similar to what we know from imaging plates diffraction systems in X-ray crystallography, can be envisioned for the near future. This progress clearly shows that electron crystallography is now on the fast track to catch up with X-ray diffraction; however, it reveals the intimate crystal structure of samples several orders of magnitude smaller than ever investigated.

In particular, due to the strong focus on the synthesis and application of nanosized materials during the last few years there is already an unsatisfied request for scientists in industry and academic research who master structural investigations by electron crystallography methods. For this reason it is highly timely to have a course on electron crystallography to prepare the next generation of crystallographers for their future task in this field.

2. PRELIMINARY SCIENTIFIC PLAN

G1. Crystallography with electrons - an overview
G2. The modern electron microscope - trends and recent achievements in electron optics and instrumentation

PART A. Structure from HREM images (not yet in the right order)

A1. Basics of HREM image formation
A2. Reconstruction of the object wave function from HREM images by the defocus variation method
A3. Crystallographic image processing of HREM images - part I and II (includes effects of optical distortions and crystal tilt, recording/digitising electron microscopy images)
A4. Combining crystallographic information from HREM images and electron diffraction

PRACTICAL SESSIONS A:
- image simulation lab (McTempas, NCEMS, EMS)
- a program for exit wave reconstruction
- Calidris make at least two practical sessions with CRISP
- VEC

PART B. Structure from electron diffraction (not yet in the right order)

B1. Introduction to the different electron diffraction techniques: SAD, CBED/LACBED, precession technique - advantages, limitations, sample requirements, typical applications
B2. Space group determination, accurate lattice parameters, structure refinement and charge density determination by CBED
B3. Solving structures by Patterson and Direct methods - theory and practice
B4. Solving structures by maximum entropy - theory and practice
B5. Structure determination using the oriented texture method - theory and practice
B6. Structure determination by gas-phase electron diffraction - theory and practice
B7. Structure refinement by taking dynamical diffraction into account
B8. Structure determination or organic compounds assisted by quantum-mechanics and X-ray powder diffraction
B9. Solving incommensurate modulated structures and composite structures from electron diffraction data
B10. Structure of nanocrystalline materials and thin-films evaluated by electron powder diffraction,
B11. Quasi-automatic solving and refining structures from SAED data

PRACTICAL SESSIONS B:
- CBED with J. Zuo's program DesktopMicroscopist
- Calidris make a practical sessions with ELD
- ProcessDiffraction for Phase identification and Rietveld Refinement
- a tutorial with CERIUS2
- the MSLS program

PART C. Complementary Methods

C1. EELS, EXELFS, ELNES - Methods for analysis of local structure, bonding and properties relationships
C2. unknown title (determining the chemical composition by EDX and phase identification by electron diffraction, databases etc.)
C3. Structure validation

PRACTICAL SESSIONS C:
- Quantitative Evaluation of EELS-Spectra for determining local structure

3. INVITED SPEAKERS:

Email messages with an invitation had gone on 27 Nov 2002. Acceptance to contribute has been received (Doug Dorset had to withdraw due to his commitments with the new employers):

Anatoly AVILOV, Moscow, Russia
Li FANG-HUA, Bejing, China
Chris J. GILMORE, Glasgow, UK
Istvan HARGITTAI, Budapest, Hungary
Jacob JANSEN, Delft, Netherlands
Christian KISIELOWSKI, LBNL, Berkeley, CA, USA
Vera KLECHKOVSKAYA, Moscow, Russia
Ute KOLB, Mainz, Germany
Joachim MAYER, Aachen, Germany
Jean-Paul MORNIROLI, Lille, France
Diana NIHTIANOVA, Sofia, Bulgaria
John C. H. SPENCE, Tempe, AZ, USA
Osamu TERASAKI, Stockholm, Sweden
Andelka TONEJC, Zagreb, Croatia
Masaki TSUJI, Kyoto, Japan
Jian-Min ZUO, Urbana, IL, USA