Feature article

Convergent-beam electron diffraction

[Beauty under blossoms] 'Beauty under blossoms' (T. Ishikawa)
The cover illustration is taken from a new book by Michiyoshi Tanaka and collaborators on electron diffraction. The book is not only scientifically sound and comprehensive, but it is visually stunning. Excerpts from the introduction and a sample of the wonderful illustrations are reprinted here – W. L. Duax, Editor

The convergent-beam electron diffraction (CBED) method is well-established in electron microscopy as an important analytical technique for the study of materials. The CBED method is widely used in materials science research for symmetry determination and strain measurements of multi-layers.

Coherent CBED, which uses a highly coherent electron source (i.e. a field-emission gun), is also a method of directly providing phase information on crystal structure factors. The coherent CBED technique produces interference fringes in the overlapping regions of CBED disks. From the relative positions of the fringes, the phase differences between two diffraction waves can be determined. The coherent CBED method can distinguish 12 space-group pairs among the 23 indistinguishable sets, and provides full dynamical methods to refine crystal structure parameters.

[Design]Since 1990, our major objective has been to develop a high-precision crystal structure refinement method based on the full dynamical theory of electron microscopy, which enables energy filtering of a CBED pattern containing both zero order Laue zone (ZOLZ) and higher order Laue zone (HOLZ) reflections. With support from JEOL and the Ministry of Education, Science, Sports and Culture of Japan, we manufactured an electron microscope (JEM- 2010FEF) equipped with a field emission gun, an objective lens with a small Cs and an omega filter. In this volume we describe examples of energy-filtered CBED patterns, a new spacegroup determination method which uses the coherent CBED technique, case studies of crystal structure refinements of CdS, LaCrO3 and h-BaTiO3 including charge density determination, simulations of Kikuchi patterns due to thermal diffuse scattering, recent studies of quasicrystals, interface analysis, which is an important application to device technology, and a future trend of the electron microscopy, an atlas of CBED symmetries for handy reference and CBED pictures for the reader’s enjoyment. The photographs were taken with JEOL electron scopes JEM-2010 and JEM-2010FEF using an imaging plate system.

Michiyoshi Tanaka, Sendai, Spring 2002