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Keywords:

  • charge ordering;
  • correlated electronic systems;
  • defects;
  • manganites;
  • transmission electron microscopy;
  • X-ray diffraction

Structural phase-transition studies employing low-temperature transmission electron microscopy and low-temperature X-ray diffraction have been carried out on nearly stoichiometric (δ = 0.01) and on off-stoichiometric (δ = 0.12) versions of La0.2Sr0.8MnO3−δ manganite. The nearly stoichiometric La0.2Sr0.8MnO3−δ undergoes a cubic to Jahn–Teller distorted C-type antiferromagnetic tetragonal phase transition at 260 K. For off-stoichiometric La0.2Sr0.8MnO3−δ the Jahn–Teller distorted tetragonal phase transition is totally suppressed and the basic perovskite lattice remains cubic but shows a charge-ordered phase. Stabilization of charge-ordered phase in the absence of cooperative Jahn–Teller distortion has been attributed to Coulomb repulsion and Hunds coupling energy. The off-stoichiometric sample shows characteristically different physical properties. This has been realized through transport, magnetic, and calorimetric measurements. A smooth crossover from variable-range hopping transport to a power-law dependence of resistivity (ρ) on temperature (T), i.e., ρ = CTα has been realized. This has been attributed to multistep inelastic tunneling through channels involving localized states around oxygen vacancy sites.