TEM Study of Two-Dimensional Incommensurate Modulation in La(2-2x)Ca(1+2x)Mn2O7 (0.6<x<0.8)

Ruddlesden-Popper n = 2 compounds La$_{2-2x}$Ca$_{1+2x}$Mn$_{2}$O$_{7}$ with 0.6 x 0.8 were synthesized by a citrate gel technique. Electron diffraction identified the presence of two-dimensional incommensurate modulations. 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 $^{o} $C. High-resolution imaging proves that the modulation in the real space is truly two-dimensional and incommensurate. Such modulation has never been observed before for La-Ca-Mn-O or any other perovskite system. The best approximation of the incommensurate superstructure is by a commensurate lattice \textbf{a}$_{x}$' = \textbf{a}$_{xt }$+ 2\textbf{a}$_ {yt}$; \textbf{a}$_{y}$' = -2\textbf{a}$_{xt}$ + \textbf{a}$_{yt}$ ; \textbf{c}' = \textbf{c}$_{t}$ (t-tetragonal I4/mmm).$_{ }$ 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 Mn$^{4+}$ and Mn$^{3+ }$ions. Structural models of the charge ordering between Mn$^{4+}$ and Mn$^{3+ }$ions are suggested.