High-resolution imaging and spectroscopy: Analysis of a n=2 Ruddlesden-Popper phase at the atomic scale
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Judith Lammer , Christian Berger , Edith Bucher , Werner Grogger
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1. Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology, Steyrergasse 17, 8010 Graz, Austria 2. Graz Centre for Electron Microscopy, Steyrergasse 17, 8010 Graz, Austria 3. Chair of Physical Chemistry, Montanuniversitaet Leoben, Franz-Josef-Straße 18, 8700 Leoben, Austria
Introduction High-resolution scanning transmission electron microscopy (HR-STEM) is a powerful tool to analyse materials at the atomic scale especially since the introduction of spherical aberration correctors, which render the recording of images with sub-Ångström spatial resolution at our ASTEM microscope possible. Moreover, we are able to receive spatially resolved chemical information of the material thanks to supplementary analytical instrumentation, such as electron energy loss spectrometers (EELS) and energydispersive X-ray spectrometers (EDXS). In this work, we show an analysis at atomic resolution of Ba1.1La1.9Fe2O7, a polycrystalline ferrite in n=2 Ruddlesden-Popper phase.
Experiments We identified the atomic structure and the crystal orientation by combining HR-STEM images (see Fig. 1) as well as their FFTs with JEMS simulations [1]: the pattern and the distances between the spots in the FFT correspond to specific orientations and lattice distances in the crystal (similar to an electron diffraction image). The contrast in high-angle annular dark field (HAADF) images is a function of the atomic number: in Fig 1. Ba (Z=56) and La (Z=57) appear brighter than Fe (Z=26), whereas O (Z=8) is not visible. It is not possible to distinguish between La and Ba in imaging methods due to their similar atomic number. Therefore we used EDX spectroscopy to acquire high-resolution elemental maps, which reveal the position of the elements in the crystal (see coloured images in Fig.1). This enables us to identify La and Ba, which are both located at the A-sites within the A3B2O7 phase. Our experiments show that La favours the 9-coordinate sites in the rock salt layer, whereas Ba prefers the 12-coordinate sites within the perovskite block (see also [2]).
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Ba1.1La1.9Fe2O7 (n=2 Ruddlesden-Popper phase) A-site
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Fig. 1: STEM HAADF: High resolution image of the RuddlesdenPopper phase in [1,0,0] orientation; four EDX maps give information about the position of the elements (La, Ba, Fe). Upper right corner: FFT of the HAADF image and simulated unit cell in [1,0,0] orientation
Conclusion
References
HR-STEM analysis reveals the atomic structure of crystalline materials, in this case the n=2 Ruddlesden-Popper phase Ba1.1La1.9Fe2O7. HAADF imaging shows the position of the Asite and B-site atoms, but can not be used for distinguishing between atoms of similar atomic number like La and Ba. EDXS is used as remedy: La and Ba are not equally distributed within the A-sites, but tend to settle either within the perovskite layer (Ba) or the rock salt layer (La).
[ 1 ] S o f t w a r e J E M S - S A A S b y D r. P. S t a d e l m a n n , http://www.jems-saas.ch/
Acknowledgements This research was supported by the Austrian Research Promotion Agency FFG (No. 853538). Financial support by the Klima- und Energiefonds within the program "Energieforschung (e!MISSION)" is gratefully acknowledged.
[2] Nicola N.M.Gurusinghe et al., 'Synthesis and characterisation of the n = 2 Ruddlesden–Popper phases Ln2Sr(Ba)Fe2O7 (Ln = La, Nd, Eu)', Materials Research Bulletin, Volume 48, Issue 9, 2013, Pages 3537-3544
Contact judith.lammer@felmi-zfe.at www.felmi-zfe.at