TEM study of two-dimensional incommensurate modulationin layered La _2-2xCa_1+2xMn₂O₇ (0.6 < x < 0.8)

Ruddlesden-Popper n = 2 compounds La_2-2xCa_1+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 [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 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 a_x′ = a_xt + 2a_yt; a_y′ = -2a_xt + a_yt; c′ = c_t (a_xt, a_yt, c _t: lattice vectors of the tetragonal I4/mmm; a_x′, a_y′, 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.