The simple infinite-aperture charge-density approximation to lattice imaging is extended to include the effects of finite aperture and objective-lens spherical aberration. Images computed using these approximations are compared with images computed by the accurate N-beam technique and with experiment in order to establish the bounds of applicability of the extended-charge-density approximations.
The calculation of amplitudes and phases of beams of electrons diffracted from thin (_ 100 rim) crystals of W4Nb26Ov7 is described. These diffraction data are used to compute 11-beam 00l electron microscope lattice images and the effects of instrument aberrations are considered. Several approximations are compared with a more exact 435-beam, two-dimensional computation, and with experimental diffraction data and lattice images. Finally, the projected charge density approximation to image contrast is evaluated.
The scattering factor for electrons is sensitive to differences in atomic bonding at low values of sin 0/2. These differences will influence the amplitudes and phases of low-order beams of electrons diffracted from crystals with large unit cells. The experimental intensities of these beams, and contrast in the corresponding lattice images, can be used to derive upper limits for the charges on constituent ions, provided the associated n-beam dynamical calculations are carried out with sufficient precision. It is shown that, in W4Nb26077, the atoms can be only partially ionized, and the bonding must have some covalent character.
Results of calculations are presented for electrons incident on barium and magnesium oxide crystals with energies in the respective ranges 0.5 to 15 eV and 0.5 to 30 eV. Comparisons are made between these computations based on a simple pseudopotential scattering matrix approach and data from LEED and total current experiments. It is concluded that the good agreement can be explained on the basis of the dominance of the crystallography of the specimen over the precise description of the individual scattering events. The coincidence of low order kinematical Bragg reflection conditions with the features is also commented upon.On prbsente ici les rksultats des calculations obtenues pour des Blectrons incidents sur des cristaux d'oxyde de barium e t magnesium, avec des 6nergies allant de 0.5 B 15 eV et de 0.5 B 30 eV. Ces calculations fond6es sur une matrice de diffusion d6duite d'un pseudopotentiel simple sont ensuite comparees et aux r6sultats de LEED et aux experiences de courant total. En conclusion, la bonne concordance des r6sultats peut etre expliqube par la dominance de la crystallographie du specimen sur la description exacte des Bvbnements de diffusion individuelle. La coincidence entre les reflections de Bragg, cinematique, d'ordre des plus bas et la systkmatique des r8sultats obtenus est cornmentee.
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