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Demie Kepaptsoglou1 Jakub Baran2 Marco Molinari2 3 Stephen Parker2 Teruyasu Mizoguchi4 Feridoon Azough4 Robert Freer5 Quentin Ramasse1

1, SuperSTEM, Daresbury, , United Kingdom
2, University of Bath, Bath, , United Kingdom
3, The University of Tokyo, Tokyo, , Japan
4, University of Huddersfield, Huddersfield, , United Kingdom
5, University of Manchester, Manchester, , United Kingdom

Recent advances in instrumentation, such as the introduction of advanced, high-resolution monochromators have allowed for new exciting experiments in the electron microscope. Spectroscopic signatures of optical and acoustical phonons, excitons and defect gap states are now accessible with an atom size probe and in tandem with high precision imaging. Here, we present results on the structure and electronic structure of thermoelectric (TE) materials for heat recovery applications, using advanced electron microscopy. Accurate information on the crystal structure and the resulting electronic properties is of paramount importance for understanding and predicting TE materials properties, as macroscopic quantities governing the material performance like the Seebeck coefficient and electronic conductivity are directly related to the electronic states in the vicinity of the Fermi level. High energy resolution scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) were used to inform theoretical predictions from density functional theory. In particular, we investigate the misfit-layered layered cobalt oxide bismuth strontium cobaltate (BSCO). We shed further light on the structure of this material, whose high efficiency makes it one of the most exciting TE oxides, highlighting the widespread occurrence of stacking faults, and finding that changes in the relative arrangements of the CoO2 and BiSrO layers can easily arise as their formation requires a very low energetic cost. We further demonstrate that Bi deficiency has a paramount importance on BSCO’s electronic, magnetic and transport properties. Atomic-layer-resolved electron energy loss spectroscopy shows how Bi vacancies lead to the hole-doping of the CoO2 layer which our theoretical modelling then demonstrates is in turn responsible for the high positive Seebeck coefficient of the material measured experimentally. Furthermore, we reveal the effect of the A-site occupancy in the structure and electronic structure in an A-site deficient perovskite system based on the Nd2/3TiO3 double perovskite. This system, another promising candidate for thermoelectric applications, has attracted significant attention due to the presence of a peculiar superstructure originating in part in cation vacancy ordering of the A-site. Using high precision atomically resolved monochromated core loss EELS measurements, acquired with an energy resolution better than 90 meV with a Nion UltraSTEM 100MC, it is possible to map individual components of the Ti L2,3 and O K near edge fine structures (ELNES). First-principles multiplet calculations are used to explain subtle changes in the ELNES, and associate them predominantly with Coulombic interactions from the A-sites. Annular Bright Field Imaging can then correlate the presence of tilting domains in the TiO6 sub lattice with these electronic structure changes observed by EELS.

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