Type of presentation: Oral

MS-5-O-2345 High-resolution STEM-EELS characterization of co-doped lanthanum niobate proton conductors

Palisaitis J.¹, Ivanova M. E.², Meulenberg W. A.², Mayer J.¹

¹Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons (ER-C), Forschungszentrum Jülich GmbH, Jülich and Central Facility for Electron Microscopy (GFE), RWTH Aachen University, Aachen, Germany, ²Institute of Energy and Climate Research (IEK-1), Forschungszentrum Jülich GmbH, Jülich, Germany

j.palisaitis@fz-juelich.de

Lanthanum niobate (LaNbO₄) is a promising material system in application as a novel proton conductor due to its combination of properties allowing to withstand high levels of humidity and CO₂ containing atmospheres [1]. The materials conductivity can be further enhanced by suitable choice of the type and amount of dopants and in particular a co-doping strategy is highly attractive as a route towards improving functional properties of the LaNbO₄ [2].
We present a detailed study of co-doping influence on the microstructure and compositional homogeneity of LaNbO₄ proton conductor using combination of high-resolution STEM-HAADF imaging and spectroscopy techniques.
Series of co-doped LaNbO₄ alloys were synthesized, where Ca, Ba or Sr acted as substitutes on La-sites, and Ge, Ti or Al on Nb-sites. As an example, 1%-Ca and 1%-Ti co-doped LaNbO₄ proton conductor with nominal formula La_0.99Ca_0.01Nb_0.99Ti_0.01O_4-δ (LCNT) is discussed in more detail in the following. A low-magnification STEM-HAADF image acquired from an as-sintered sample is shown in Fig. 1. LCNT is predominantly composed of low temperature monoclinic, randomly oriented and well packed large grains showing hexagonal shapes and stress-induced stripy patterns. Strong elemental contrast STEM-HAADF images provided indication for curved shape secondary phase grains present in the host matrix, like grains denoted by letter ‘S’ in Fig. 1. Core-loss EELS spectra revealed the compositional nature and established the presence of a dopant-rich phase in LCNT (see Fig. 2). Furthermore, valence EELS spectroscopy confirmed the same chemical nature for all studied secondary phase grains (not shown). Fig. 3 shows a high-resolution STEM-HAADF image together with the corresponding electron diffraction pattern acquired from one of the ‘S’ grain oriented along a low index zone axis. The crystal structure of the secondary phase grain is highly ordered, layered and defect-free in the observed projection. The layering was characterized by a representative periodicity of ~12.92 Å perpendicular to the layers. High spatial resolution core-loss EELS line profiles of the Ca-L₂₃, Ti-L₂₃, and La-M₄₅ absorption edge intensities are shown in Fig. 4. The ‘S’ grains contain La-rich layers (also accommodating some amount of Ca) which are separated by Ti-Ca rich layers accommodating some amount of La. By correlating experimental data with crystal structure models, we identified that the secondary phase grains possess the lanthanum titanate (La₂Ti₂O₇) crystal structure with substantial amounts of Ca incorporated into them, where Ca partly substitutes La.

References:
[1] R. Haugsrud, T. Norby, Nature Materials 5 (2006) 193.
[2] M. Ivanova, S. Ricote, W. Meulenberg, R. Haugsrud, M. Ziegner, Solid State Ionics 213 (2012) 45.

Financial support by the Helmholtz-Society in the framework of the Portfolio Project MEMBRAIN is gratefully acknowledged.

Fig. 1: Overview STEM-HAADF image acquired from La_0.99Ca_0.01Nb_0.99Ti_0.01O_4-δ sample showing the presence of secondary phase grains denoted by ‘S’.	Fig. 2: Core-loss EELS spectra recorded from the host matrix and the ‘S’ grain.
Fig. 3: High-resolution STEM-HAADF image acquired from the ‘S’ grain together with corresponding electron diffraction pattern in the inset.	Fig. 4: High spatial resolution core-loss EELS elemental line profiles plotted as a function of probe scanning position perpendicular to the layers in the ‘S’ grain.