TEM study of two-dimensional incommensurate modulationin layered La 2-2xCa1+2xMn2O7 (0.6 < x < 0.8)

Leonid A. Bendersky, Ian D. Fawcett, Martha Greenblatt

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Abstract

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 [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 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.

Original languageEnglish (US)
Pages (from-to)5304-5310
Number of pages7
JournalChemistry of Materials
Volume16
Issue number25
DOIs
StatePublished - Dec 14 2004

All Science Journal Classification (ASJC) codes

  • Chemistry(all)
  • Chemical Engineering(all)
  • Materials Chemistry

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