An experimental atomic resolution analysis of an undoped Σ5 36° [001] tilt grain boundary in SrTiO3 shows that the structure contains incomplete oxygen octahedra. These incomplete octahedra act as effective oxygen vacancies and lead to a fixed, positive boundary charge. Annealing the boundary in the presence of MnO2 does not change the atomic structure of the boundary plane, and results in a high concentration of Mn3+ (acceptor) enrichment at the specific Ti4+ locations in closest proximity to the effective oxygen vacancies. This result can be explained in terms of standard charge compensation models and indicates that the formation of electrical barriers at oxide grain boundaries may be influenced by the atomic structure of the boundary plane.
Grain boundaries in oxide materials such as electroceramics, ferroelectrics, and high-T c superconductors are known to dominate their overall bulk properties. The critical first step in a fundamental understanding of how they control the properties of the material is a determination of the atomic structure of the boundary. While this determination has traditionally been performed by transmission-electron microscopy, the images that are generated are only a two-dimensional projection of the atomic columns in the grain-boundary core. In addition, as the images are least sensitive to light elements, such as oxygen, the complete three-dimensional boundary structure is particularly difficult to determine. Employing electron-energy-loss spectroscopy in a scanning transmission-electron microscope, it is possible to obtain an oxygen K-edge spectrum that contains information on the three-dimensional electronic structure of the boundary. Using the multiple-scattering methodology, originally developed for x-ray absorption near-edge structure, this can be directly related to the local threedimensional atomic structure. Contained in the spectrum is therefore all of the information needed to investigate the atomic scale structure-property relationships at grain boundaries. The application of the technique is demonstrated here for the 25°͓001͔ symmetric tilt boundary in SrTiO 3 .
: Determining the three-dimensional atomic structure of grain boundaries is a crucial first step toward understanding how these defects control the overall bulk properties of materials. In this report we discuss the correlation of experimental atomic resolution Z-contrast images and electron energy loss spectrometry (EELS) to achieve this goal. Initial structural analysis is afforded through empirical bond-valence potentials. This structure is then refined using multiple scattering analysis of the energy loss spectra. These techniques are demonstrated in the analysis of a 27 degrees MgO [001] tilt grain boundary. Through this analysis, we were able to determine specific atomic locations of Ca dopants found present at this grain boundary.
We describe the use of bond–valence analysis to investigate the segregation of calcium atoms at an MgO [001] tilt grain boundary. For small deviations away from the equilibrium metal‐oxide bond length, the bond–valence parameter approximates well to a Born–Mayer‐type pair potential. Therefore, by starting with a structural model determined from an atomic resolution Z‐contrast image, compositional and cation valence state information obtained from electron energy loss spectroscopy (EELS) can be incorporated into a comprehensive model for the grain boundary. For the boundary under investigation here, it is found that specific sites for preferential segregation can be identified, resulting in 0.3–0.4 monolayers of calcium in the boundary plane.
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