The structural modulation of La1-xCaxMnO3 has a uniform periodicity at any doping x > 0.5

Authors:
  • JLJ. C. Loudon
    J. C. Loudon
  • SCS. Cox
    S. Cox
  • JAJ. P. Attfield
    J. P. Attfield
  • PLP. B. Littlewood
    P. B. Littlewood
  • PMP. A. Midgley
    P. A. Midgley
  • NMN. D. Mathur
    N. D. Mathur
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Abstract
The so-called charge ordered stripe phases seen at low temperatures in the pseudo-cubic perovskite manganites represent a genre of phases whose coexistence over a range of length scales has rejuvenated the field. It has hitherto been believed that the associated modulations must possess a high degree of local registry with the underlying crystal lattice. Therefore, modulations at arbitrary doping levels have been considered to contain large local fluctuations in periodicity. Here, we combine low temperature electron microscopy of polycrystalline La1-xCaxMnO3 (0.5 < x < 0.71) with computer simulations of a model system. Surprisingly, we find that on a sufficiently small lengthscale (< 50 nm) the modulation wavevector does not vary when the size of the region sampled approaches unit cell dimensions. Therefore the observed modulations cannot be described as a fine mixture of integer period sub-units delineated by discommensurations. Our discovery of uniform periodicity at any doping (x > 0.5) rules out the existence of just two motifs such as stripes characterised simply by the presence or absence of charge. This throws doubt upon previous interpretations of modulations in the manganites.

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