Effect of doping with iron on the charge ordering in La0.33Ca0.67Mn1-yFeyO3 (y=0, 0.05) manganites

The formation and specific features of the superstructure in La0.33Ca0.67Mn1-y Fe (y) O-3 (y = 0, 0.05) manganites doped with iron are investigated using transmission electron microscopy. The electron diffraction patterns of the manganites are studied in the temperature range 90-300 K, and the high-resolution electron microscope images recorded at temperatures of 91-92 K are analyzed. In both manganites, the structural transition that is accompanied by the formation of the superstructure and which is directly observed from the appearance of additional peaks in the electron diffraction patterns occurs at a temperature that is in close agreement with the charge ordering temperature T (CO) determined from the temperature dependences of the magnetization M(T). In the temperature range 90 < T < 200 K, the undoped compound has a commensurate superstructure characterized by the vector q = 1/3a* and triple the unit cell << 3a x b x c >> (where a approximate to b approximate to root 2a(p), c approximate to 2a(p) , and a(p)similar to 3.9 angstrom is the lattice parameter of a simple perovskite). The doping with iron (5 at. %) brings about a decrease in the charge ordering temperature T (CO) by 50 K and the formation of an incommensurate structure for which the magnitude of the vector q is smaller by approximately 15%. The unit cell of the superstructure in the iron-doped compound is not triple the unit cell but involves defects of ordering, such as quadrupling of the unit cell, numerous translations by a(p)root 2 along the a direction, and dislocation-type defects in the stripe structure of the charge ordering. These pseudoperiodic defects lead to a decrease in the magnitude of the vector q and are responsible for the incommensurability of the structure. A decrease in the charge ordering temperature T-CO due to the doping with iron and the incommensurability of the superstructure correlate with the change in the concentration of Mn3+ Jahn-Teller ions as a result of their replacement by Fe3+ non-Jahn-Teller ions.