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Polar ordering induced by oxygen vacancies

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Atomic resolution Z-contrast image shows a (LaFeO3)2/(SrFeO3) superlattice film grown on SrTiO3 substrate. The oxygen K-edge electron energy loss signal from the film overlaid on the image shows periodic oscillations indicating partial ordering of oxygen vacancies. Only the region of the film which shows vacancy ordering shows polar behavior.
A combination of scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and density functional theory (DFT) calculations show that it is possible to achieve polar order in a superlattice made up of two non-polar oxides by means of oxygen vacancy ordering. While complex oxide films often have finite concentrations of oxygen vacancies that are commonly thought to be detrimental, this work shows that vacancies can instead lead to new functionalities and possibly new applications.

Z-contrast STEM images show the presence of a polar displacement of magnetic Fe ions in a nanometer-scale region of a (LaFeO3)2/(SrFeO3) superlattice film. DFT calculations show that commonly studied order parameters (epitaxial strain, octahedral tilts) have negligible effect on the displacement of the Fe ions, whereas the introduction of oxygen vacancies leads to finite polar displacements. These results are confirmed using EELS, which shows oxygen vacancy ordering in the region where polar displacements are observed, and absence of vacancy ordering outside of that area. 

R. Mishra, Y.-M. Kim, J. Salafranca, S. K. Kim, S. H. Chang, A. Bhattacharya, D. D. Fong, S. J. Pennycook, S. T. Pantelides, and A. Y. Borisevich,  “Oxygen-vacancy-induced polar behavior in (LaFeO3)2/(SrFeO3) superlattices,” Nano Lett. 14, 2694 (2014).    [doi:10.1021/nl500601d]  

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