文档介绍：International Tables for Crystallography (2006). Vol. B, Chapter , pp. 276–345.
. Electron diffraction and electron microscopy in structure determination
BY J. M. COWLEY,,, AND D. L. DORSET
. Foreword (J. M. COWLEY) aberrations of cylindrical ic lenses have, to date, limited the
resolution of electron microscopes to the extent that the least
Given that electrons have wave properties and the wavelengths lie resolvable distances (or ‘resolutions’) are about 100 times the
in a suitable range, the diffraction of electrons by matter is electron wavelength. However, with microscopes having a
completely analogous to the diffraction of X-rays. While for resolution of better than 2 A˚ it is possible to distinguish the
X-rays the scattering function is the electron-density distribution, individual rows of atoms, parallel to the incident electron beam, in
for electrons it is the potential distribution which is similarly peaked the principal orientations of many crystalline phases. Thus
at the atomic sites. Hence, in principle, electron diffraction may be ‘structure images’ can be obtained, sometimes showing direct
used as the basis for crystal structure determination. In practice it is representation of projections of crystal structures [see IT C (1999),
used much less widely than X-ray diffraction for the determination Section ]. However, plications of dynamical scattering
of crystal structures but is receiving increasing attention as a means and of the coherent imaging processes are such that the image
for obtaining structural information not readily accessible with intensities vary strongly with crystal thickness and tilt, and with the
X-ray- or neutron-diffraction techniques. defocus or other parameters of the imaging system, making the
Electrons having parable with those of the interpretation of images difficult except in special circumstances.
X-monly used in diffraction experiments have energies of F