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

Abstract

In this paper, we report on a synthesis procedure, structural and electrical properties of BaErMn2O5 and BaErMn2O6, A-site double perovskites having layered arrangement of Ba and Er cations. These materials belong to a family of BaLnMn2O5+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 BaErMn2O5+δ, from a point of view of oxygen storage technology. Electrical conductivity of BaErMn2O5 is of the order of 10−4 S cm−1 at room temperature and shows activated character on temperature with activation energy Ea=0.30(1) eV. Positive sign of Seebeck coefficient for this material indicates holes as dominant charge carriers. Oxidized BaErMn2O6 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.