Atomic Scale Characterization of Oxygen Vacancy Segregation at SrTiO 3 Grain Boundaries
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Atomic Scale Characterization of Oxygen Vacancy Segregation at SrTiO3 Grain Boundaries R.F. Klie and N. D. Browning Department of Physics (M/C 273), University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607-7059. USA.
ABSTRACT We have examined the structure, composition and bonding at an un-doped 58° [001] tilt grain-boundary in SrTiO3 in order to investigate the control that the grain boundary exerts over the bulk properties. Room temperature and in-situ heating experiments show that there is a segregation of oxygen vacancies to the grain boundary that is increased at elevated temperatures and is independent of the cation arrangement. These measurements indicate that the widely observed electronic properties of grain boundaries may be due to an excess of mobile oxygen vacancies that cause a highly doped n-type region in the close proximity ( ≈ 1 unit cell) to the boundary. These results are shown to be consistent with both theoretical models and lower resolution chemical analysis.
INTRODUCTION Oxygen vacancies in bulk materials and at internal interfaces have long been known to be responsible for many interesting mechanical and electronic properties. In the case of one of the most widely applied classes of functional materials, the perovskite oxides, extensive microstructural evaluations have been performed in an effort to understand the structure-property relationship [1, 2]. Although many of these analyses were performed using high resolution microscopy techniques, the results were not obtained on the atomic level. Therefore, interpretations had to be made in the context of bulk defect chemistry models and macroscopic transport measurements [3]. As such, the fundamental atomic scale origins of the properties could not be investigated. In this report we will describe novel techniques in the scanning transmission electron microscope (STEM) that can be used to analyze the atomic scale structure-property relationships in these, and other materials, both at room and elevated temperatures. In particular, by using correlated Z-contrast imaging and electron energy loss spectroscopy (EELS), the structure, composition and bonding can all be characterized directly on the atomic scale [4-8]. This ability to correlate precisely the atomic features giving rise to the materials properties means that these experiments can be used to accurately describe the atomic scale defect chemistry. Furthermore, the heating experiments allow the dynamics of oxygen vacancies [2, 9] to be investigated at temperatures close to the standard operating conditions for many high temperature oxides. Although a wide range of materials can be analyzed by these methods (dielectrics, electronic and ionic conductors, catalysts, semiconductors and CMR materials), here we will concentrate on the atomic scale classification of oxygen vacancy structures at grain boundaries in model electronic perovskites. AA1.7.1
STEM METHODS The experimental results that are presented in this paper are all obtained using the JEOL 2010F STEM/TEM microsc
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