Enhancement of localization phenomena driven by covalency in the SrBiMn1.75Ti0.25O6 manganite

Manganites are materials that show remarkable phenomena related to charge orbital ordering (CO/OO) and it is extremely important to understand the fundamental nature of this behaviour. This paper reports on the structural, electronic and magnetic behaviour of the new SrBiMn1.75Ti0.25O6 manganite and the dependence of these properties with temperature. A detailed structural analysis has been carried out by electron, X-ray, neutron diffraction between 4 and 700 K. The electron diffraction patterns obtained at room temperature (RT) evidence that the average structure (a∼b∼2ap and c ∼ 2ap) presents a modulation that doubles the a and c lattice parameters. A very high charge ordering (CO) transition temperature of 510 K, similar to that found for the non-doped material, SrBiMn2O6, is observed. Above this temperature the symmetry changes from orthorhombic to tetragonal. The conductivity and susceptibility data show a strong interdependence with the structural transition. The activation energy decreases from 0.24 eV to 0.12 eV below and above 510 K, respectively. On the other hand, the high value of the magnetic moment (μeff = 9.34 μB), observed between 300 and 500 K, is interpreted in terms of the existence of Mn4 magnetic clusters. Finally, extra maxima appearing in neutron diffraction patterns at lower temperatures are interpreted considering the stabilization of AFM interactions, consistent with an A-type magnetic ordering. The results are discussed in the framework of charge ordering/orbital ordering and cluster models.

Enhancement of localization phenomena driven by covalency in the BiSrMn1.75Ti0.25O6 manganite. The CO/OO phenomena and magnetic clusters (Mn4) stabilization at temperatures up to 520 K in BiSrMn1.75Ti0.25O6 are connected to a structural transition. The observed enhancement of electronic localization is interpreted considering covalent effects of Ti4+ and Bi3+ cations as the main driving force.Figure optionsDownload as PowerPoint slideHighlights
► The structural, electronic and magnetic behaviour of the new SrBiMn1.75Ti0.25O6 manganite is reported.
► A structural transition above 520 K takes place, coinciding with relevant changes in the transport properties.
► The electronic behavior is interpreted in terms of a remarkably high orbital and charge ordering temperature and cluster models.
► The observed features are explained considering a scenario in which bonds covalence is enhanced by the Bi3+ and Ti4+ cations.