Exact Atom Positions by Electron Microscopy?

Abstract

Direct imaging of structures is possible by high-resolution electron micros­copy (HREM). This analytical method is therefore of special attraction in the field of structural chemistry. X-ray crystallography has been extremely successful for the determination of crystal structures, suggesting that there was no necessity to use high-resolution electron microscopy for that pur­pose. However, crystals that are thousands times smaller in size than that needed for single crystal X-ray measurements can be studied by EM since electrons interact much stronger with matter than X-rays. On the other hand, the strong scattering of electrons causes severe problems to obtain quasi-kinematical data. Therefore it is always necessary to take great care on the proper experimental conditions in quantitative EM. It was shown more than one decade ago that precise heavy atom positions for metal oxides could be determined if the technique of crystallographic image pro­cessing (CIP) is combined with EM [1]. Since then, the CIP technique for structure determination has been applied successfully to other metal oxides.