Ruddlesden-Popper n = 2 compounds La2-2xCa1+2xMn 2O7 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 two-dimensional incommensurate modulation, and the sets are rotated with respect to each other around  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 two-dimensional 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 ax′ = axt + 2ayt; ay′ = -2axt + ayt; c′ = ct (axt, ayt, c t: lattice vectors of the tetragonal I4/mmm; ax′, ay′, c′: 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 Mn 3+ ions are suggested.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Materials Chemistry