Synthesis, crystal structure and electrical properties of A-site cation ordered BaErMn₂O₅ and BaErMn₂O₆

In this paper, we report on a synthesis procedure, structural and electrical properties of BaErMn₂O₅ and BaErMn ₂O₆, A-site double perovskites having layered arrangement of Ba and Er cations. These materials belong to a family of BaLnMn ₂O_5+δ oxides, which up to now were successfully synthesized for Ln=Y and La-Ho lanthanides. Up to our knowledge, this is the first report on the successful synthesis of BaErMn₂O₅ and BaErMn₂O₆, yielding > 95 wt% of the considered compounds. Structural characterization of the materials is given at room temperature, together with in situ XRD studies, performed during oxidation of BaErMn₂O₅ in air, at elevated temperatures up to 500 °C. A complex structural behavior was observed, with oxidation process of BaErMn₂O₅ occurring at around 300 °C. The oxidized BaErMn₂O₆ shows a structural phase transition at about 225 °C. Results of structural studies are supported by thermogravimetric measurements of the oxidation process, performed in air, as well as reduction process, preformed in 5 vol% of H₂ in Ar. Additionally, isothermal oxidation/reduction cycles were measured at 500 °C, showing interesting properties of BaErMn₂O_5+δ, from a point of view of oxygen storage technology. Electrical conductivity of BaErMn₂O ₅ is of the order of 10^-4 S cm^-1 at room temperature and shows activated character on temperature with activation energy E_a=0.30(1) eV. Positive sign of Seebeck coefficient for this material indicates holes as dominant charge carriers. Oxidized BaErMn₂O ₆ possesses much higher electrical conductivity, almost 0.2 S cm ^-1 at room temperature. Additional, about 10-fold increase of electrical conductivity, occurring in the vicinity of 225 °C for this material, can be associated with phase transition from charge/orbital-ordered insulator COI(CE) to paramagnetic metal PM phase. The highest conductivity for BaErMn₂O₆ was measured near 500 °C and is almost equal to 40 S cm^-1, while negative sign of Seebeck coefficient can be associated with electrons being dominant charge carriers.