Annual report of International Union of Crystallography Commission on Electron Diffraction


Written Feb 2005 by John Spence

The highlight of 2004 has been the School for Electron Crystallography, held at Erice, in Sicily, June 9 -20 at the NATO advanced study institute. This was capably organisec by T. Weirich, J.Labar, and X. Zou. About 90 participants and lecturerers attended over a week of lectures on all aspects of electron crystallography, including microdiffraction, atomic-resolution imaging and new approaches to solving nanocrystalline structures by electron diffraction. Other topics included polymorphism, phasing electron diffraction data, multiple scattering, symmetry determination, lab and software sessions, atomic resolution imaging, electron diffraction from organics and zeolites, charge-density measurement, lattice parameters, and gas phase diffraction. A recurring theme was the ability to treat small crystals whose size prevents the use of X-ray diffraction, and the power of imaging for the study of defects.

The development of time-resolved electron diffraction continued with the first US National Workshop on Ultrafast Electron Microscopy at Lawrence Livermore Laboratories on April 16-17, 2004. 51 participants from universities, National Labs and companies attended two days of lectures on fast imaging and diffraction with high energy electron beams, including those planning to use the electron accelerators for synchrotrons directly for this pupose. The current instruments at Caltech, Brown, U. Toronto and Florida State were reviewed, together with new ones planned at Michigan State, Univ of Illinois and Berkeley. The pioneering imaging instrument in Berlin will soon move to Livermore. Talk topics included the design of photocathodes, electron lenses, detectors and space-charge limitations and pulse compression. The much larger cross-section for electron scattering than for X-ray was emphasized, while the source brightness of field-emission electron sources is known to be brighter than that of current generation synchrotron-undulator systems. The attainment of 700 femtosecond electron diffraction results in single-shot by Cao et al at Florida was discussed, as was Zuo's iterative phasing of continuous diffraction data by the Fienup-Gerchberg-Saxton method. Applications in materials science and biology were reviewed.

The Gordon Conference on Charge-density measurements held from July 4 2004 included a morning session on electron diffraction methods for accurate charge-density measurement. The amplification of sensitivity at low angles in electron scattering over X-rays was emphasized (due to the Mott-Bethe relationship), while the ability to obtain extinction-free measurements by using an electron probe smaller than one mosaic block was also described, with applications to several oxides.

About 20 students, postdocs and industrial researchers attended a week-long school in electron diffraction at the National Center for Electron Microscopy at Lawrence Berkeley Laboratory, Berkeley, Ca., USA, starting on April 19, 2004. Lectures by L. Marks, J. Zuo, W. Sinkler, Spence, Dahmen, Eades and Kilaas covered all aspects of electron crystallography, including CBED, SAD, Diffuse scattering, powder patterns for phase identification, combining SAD with HREM, and basic theory, from dynamical theory, Bloch waves, channelling and multislice, to direct methods. Special topics included diffractive imaging and the precession camera. Afternoons were devoted to practical classes and computer use for simulations. One set of programs for most electron crystallography purposes, now executable on the web for all to use, can be found at http://emaps.mrl.uiuc.edu/ . A similar school may be held next year.