A NEW APPROACH TOWARDS MEASURING STRUCTURE FACTORS AND VALENCE-ELECTRON DISTRIBUTION IN CRYSTALS WITH LARGE UNIT CELLS

By focusing a probe of fast electrons above or below the specimen leve l in convergent-beam electron diffraction, shadow images of many refle ctions are recorded in one exposure. For wedge-shaped crystals, this p rocedure provides a means of recording thickness or Pendellosung fring es of many reflections simultaneously, which ensures that the experime ntal conditions with respect to thickness variations and crystallograp hic direction of the incident beam are the same for all reflections. D isturbing defects within the illuminated area of the specimen are imme diately revealed in the shadow images of the different reflections. Th e approach is introduced by using strontium titanate and silicon as ex amples. As a case study, the technique is applied to the dense (00l) r eciprocal-lattice row of the superconductor YBa2Cu3O7 to address charg e transfer and electron-hole distribution by determining the amplitude s and phases of the 001 and 002 reflections. By this approach, the abs olute values of the structure factors are obtained from the very thin regions where the kinematical theory applies. Based on these approxima te absolute values, the signs of the structure factors in centrosymmet rical crystals are determined by comparing dynamically calculated and observed intensities in somewhat thicker crystal regions for different combinations of signs. As a third step, the structure factors are ref ined by comparing the calculated and experimental intensity profiles w ithin the entire observed thickness range.