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 wa reconstructed by crystallographic image processing (CIP). In the reconstructed image, showing 4bm symmetry, all metal atom were clearly seen with correct contrast, whereas in the original micrograph the fourfold symmetry was lost and only sore areas could be interpreted in terms of an atomic arrangement. Defocus, astigmatism and electron beam and crystal tilt wer evaluated by CIP, and it was found that the distortion in this case was mainly due to beam tilt and crystal tilt. After correctinl for the distortions, a calculated map was compared to the structure of an apparently isomorphous compound T1Nb7OI~ previously solved by X-ray diffraction. The atomic positions agreed to within + 6 pm. A simulated image with electron bear tilt was in good agreement with the experimental one.