Structural and Morphological Changes Associated with Charge Ordering in La_0.2Ca_0.8MnO₃

Both structural and morphological changes associated with charge ordering in perovskite manganese oxide, La_0.2Ca_0.8MnO₃, were studied by electron microscopic observations with imaging plates. Two types of long-period structures were observed at around 107 K, i.e., the structure with about 4-fold periodicity to the unit lattice distance of the room-temperature phase and the structure with much larger periodicity such as 23-fold. These long-period structures were suggested to be formed by a similar mechanism due to charge ordering of manganese ions, since they showed some crystallographic and morphological similarities. Superlattice reflections of a near 4-fold structure were observed as a function of temperature. They were found to become weak diffuse scattering just before the disappearance, and the peak positions moved slightly with heating. Considering those features, the kinetics of the charge ordering in this oxide was discussed.