STRUCTURE DETERMINATION AND CORRECTION FOR DISTORTIONS IN HREM BY CRYSTALLOGRAPHIC IMAGE-PROCESSING

A heavily distorted high resolution electron microscopy (HREM) image of a cesium-niobium oxide-fluoride sample was reconstructed by crystallographic image processing (CIP). In the reconstructed image, showing 4bm symmetry, all metal atoms were clearly seen with correct contrast, whereas in the original micrograph the fourfold symmetry was lost and only some areas could be interpreted in terms of an atomic arrangement. Defocus, astigmatism and electron beam and crystal tilt were evaluated by CIP, and it was found that the distortion in this case was mainly due to beam tilt and crystal tilt. After correcting for the distortions, a calculated map was compared to the structure of an apparently ismorphous compound Approximately TlNb//7O//1//8, previously solved by X-ray diffraction. The atomic positions agreed to with plus or minus 6 pm. A simulated image with electron beam tilt was in good agreement with the experimental one.