The atomic structure of grain boundaries in SrTiO3 and other electroceramic materials governs a variety of macroscopic electrical properties, including technologically important effects, such as nonlinear I-V characteristics. An understanding of the relationship between the atomic structure and the electrical properties of individual grain boundaries requires an atomic scale investigation of both the composition and chemical bonding at the boundaries. For the first time, such a detailed structural model has been obtained for a SrTiO3 grain boundary. This was achieved by applying atomic-resolution Z-contrast imaging to locate the cation columns at the boundary in combination with atomic resolution electron energy loss spectroscopy (EELS), which examined light element coordination with atomic resolution. The work was carried out on a tilt boundary in a SrTiO3 bicrystal and was made possible by simultaneous use of ultrasensitive, parallel detection EELS and atomic-resolution Z-contrast scanning transmission electron microscopy (STEM).
Figure 1 shows a Z-contrast STEM image of a 25°  tilt boundary in SrTiO3 before and after maximum entropy image processing. The brighter spots in the image correspond to the (strongly scattering) Sr columns, and the less bright spots correspond to the lighter Ti-O columns. The accuracy of ±0.2 Å without the use of a reference atomic model, and complementary inofmation from the atomic resolution EELS measurements revealed that octahedral Ti-O coordination is maintained even at the boundary plane. By combining the EELS and the Z-contrast STEM information, and refining the postions of the oxygen columns by bond-valence sum calculations, a detailed model of the boundary structure was constructed, as shown in Fig. 2. Of particular interest in these results are the half-occupied Sr dopant atoms, which have asignificant effect on the electrical properties of these materials. The formulation of a detailed model provides a basis for initiating detailed theoretical investigations of grain boundary structure and chemistry. As structural studies are performed on doped and undoped grain boundaries and correlated with electrical characterization, it should now be possible to elucidate the relationships between microscopic grain-boundary structure and physical properties in electroceramic materials.