Influence of internal electric field on the transport properties of the magnetoplumbite system BaFe_12-xMn_xO₁₉

Abstract

Measurement of the room temperature conductivity and Seebeck coefficient for the magnetoplumbite system BaFe12-xMnxO19, 0<or=x<9, shows that Mn substitution increases the p-type conductivity caused by an enhancement of hole concentration, presumably Fe4+ or Mn4+ ions. This observation supports the deduction from neutron diffraction data that the Mn in the tetrahedral sites 4fiv of the spinel blocks is Mn2+. Such a formal-valence distribution is shown to be possible because of a lack of local charge neutrality in the structure. The intergrowth of (BaFe6O11)2- R blocks and (Fe6O8)2+ S blocks creates an internal electric field parallel to the c axis that shifts the redox potentials at 4e(1/2) sites relative to those at 4fiv sites. Measurement of the temperature dependence of the conductivity gives Arrhenius plots with two activation energies; E1 for T<Tc and E2 for T>Tc, where Tc is the Curie temperature. E1 approximately=0.37 eV appears to be a motional enthalpy which is essentially independent of x, but E2>E1 at x=0 changes to E2<E1 at larger x.