TEM Study of Two-Dimensional Incommensurate Modulationin Layered La2-2xCa1+2xMn2O7 (0.6 < x < 0.8)

Ruddlesden-Popper n = 2 compounds La2-2xCa1+2xMn2O7 with 0.6 <=x<= 0.8 were synthesized by a citrate gel technique and studied by transmission electron microscopy. Electron diffraction identified the presence of eight satellite reflections around fundamental spots of the basic tetragonal I4/mmm structure. The satellite reflections belong to two sets of twodimensional incommensurate modulation, and the sets are rotated with respect to each other around [001] by ~29°. The modulation is sensitive to electron-beam irradiation and rapidly degrades under a focused beam. In situ heating experiments indicated the existence of possible tetragonal-to-incommensurate phase transition around 350 °C, although the presence of the modulation persists up to 500 °C. High-resolution imaging proves that the modulation in the real space is truly twodimensional and incommensurate and not a combination of one-dimensional commensurate modulations. Such modulation has never been observed before for La-Ca-Mn-O or any other perovskite system. The best approximation of the incommensurate superstructure is by a commensurate lattice axʹ = axt + 2ayt; ayʹ = -2axt + ayt; cʹ = ct (axt, ayt, ct: lattice vectors of the tetragonal I4/mmm; axʹ, ayʹ, cʹ: lattice vectors of the proposed approximant). Such superstructure could be satisfied with a 4:1 two-dimensional ordering in a (001) plane. For the studied compositions, such 4:1 ordering is plausibly described by charge ordering between Mn4+ and Mn3+ ions. Structural models of the charge ordering between Mn4+ and Mn3+ ions are suggested.