CREMS 2021 Corneliu Ghica

Revealing chemical ordering in BaLnCo2O6-δ double perovskites by atomic-resolution analytical TEM C. Ghica1, M. C. Istrate1, S. L. Wachowski2, I. Szpunar2, A. Mielewczyk–Gryń2, M. Gazda2 1 National Institute of Materials Physics, 077125 Măgurele, Romania 2 Gdańsk University of Technology, 80-233 Gdańsk, Poland Barium lanthanide cobaltites, BaLnCo2O6-δ (Ln = La, Pr, Nd, Sm, Gd, Tb or Dy), are double perovskites intensively studied for applications in electrochemical and electrocatalytic devices. Several polymorphs are observed, among which the cubic (SG Pm-3m), the tetragonal (SG P4/mmm) or the orthorhombic (Pmmm) structures. The double perovskite structure consists in a layered structure determined by the Ba and Ln ion ordering on alternating A-type positions along the c axis, leading to a doubling of the corresponding cell parameter. Analytical TEM/STEM investigations have been performed on a Cs probe-corrected JEM ARM 200F instrument provided with a bottom-mounted Gatan Quantum EEL Spectrometer with the aim of refining the atomic structure of the synthesized BaLnCo2O6-δ double perovskites while revealing different structural ordering variants likely to occur, such as Ba/Ln cation ordering or oxygen vacancy ordering. The analyzed crystal grains typically contain structural domains for which the SAED patterns show additional spots in halfway positions, thus indicating the doubling of the b and/or c lattice parameter with respect to the simple perovskite. The HRTEM micrograph in figure 1 contains two neighboring domains denoted A and B, where intensity modulations can be noticed along the [001] and [011] directions corresponding to a doubling of the interplanar distance to 0.75 nm and 0.54 nm, respectively. The intensity modulation along [001] with a 0.75 nm period can be associated with the Ba/Pr cation ordering along the c-axis. We have used HRTEM image simulation by Crystal Kit and Mac Tempas to demonstrate that the observed intensity modulation along [011] is associated to the oxygen positions, pointing to a process of oxygen vacancies ordering. To further confirm the cation ordering, we have analyzed the samples in the STEM-EELS spectrum imaging mode. A typical example is presented in figure 2 for the BaLaCo2O6-δ sample. The atomic resolution elemental mapping clearly shows the Ba/La ordering, with the formation of the double perovskite cell with a lattice parameter c=0.77 nm. Acknowledgement CG and MCI acknowledge the financial support from Romanian Ministry of Research and Innovation in the frame of PN19-03 and POC 332/390008/29.12.2020-SMIS 109522 projects. References [1] S. L. Wachowski, I. Szpunar, M. H. Sørby, A. Mielewczyk–Gryń, M. Balaguer, C. Ghica, M. C. Istrate, M. Gazda, A. E. Gunnæs, J. M Serra, T. Norby, R. Strandbakke, Acta Materialia 199, 297 (2020)