Aspects of electronic transport in YBaCo4O7+δ pellets

YBaCo4O7+δ powders were obtained by standard solid state reaction und their structural, morphological and electrical properties carefully analyzed. The X-ray powder diffraction patterns showed reflexes corresponding to a pure hexagonal structure (space group P63mc). The lattice parameters resulted to be very close to those reported in the literature for high-quality samples. Raman spectra at room temperature allowed for identifying bands associated with vibrating modes of CoO4 and Y2O6 in tetrahedral and octahedral coordination, respectively. Additional bands, which seemed to stem from CoO in octahedral coordination, were also clearly identified. The dependence of the resistivity on temperature showed a semiconducting-like behavior and no indication of structural phase transition was observed up to temperatures as low as 20 K. The electronic transport mechanism in this material was analyzed within the framework of standard models as thermal activation, polaronic-type conductivity or Mott variable-range hopping. Contrary to some reports in the literature in which thermal activation was reported to be the main transport mechanism, careful analysis of the obtained resistivity data (this work) favored the variable-range hopping conduction model. Certainly, the experimental data recorded in a wide temperature range were well described by the function ρ(T) = ρ0exp[(T*/T)1/4]. The fit procedure yielded a temperature scale T* ∼ 106 K, similar to that found in other transition metal oxides. This parameter, in turn, allowed for estimating the density of states at the Fermi level N(EF) for this compound.

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