Electron-microscopy and electron-diffraction study of order-disorder phenomena in the intercalate C₆₀(I₂)_2−x

Abstract

Single crystals of iodine-intercalated C₆₀ have been examined by transmission electron microscopy. Tilting series of Electron Diffraction Patterns (EDP) confirm that it has a simple hexagonal structure with c/a≈1 in accordance with the results of X-ray diffraction, which allows higher precision c/a=1.0024. Cooling and heating experiments were performed in the vacuum of the microscope to investigate the order — disorder phenomena. High-Resolution Electron Microscopy (HREM) images and electron diffraction patterns reveal that the c/2-displacement along the c-direction plays an important role in the transition from the simple hexagonal structure into the fcc structure when iodine is lost from the specimen or “vice versa”. The orientation relationship between the intercalate and the C₆₀ structure is [01$\overline {\text{1}}$10]_h∥[1$\overline {\text{1}}$10]_fcc and (001)_h∥(110)_fcc. A model is proposed to interpret the features observed in EDP and HREM consistently.