Measurement of oxygen in oxide materials

Oxygen features eminently in materials that are of considerable technical importance. Such materials include the large group of electroceramic materials comprising dielectrics, ferroelectrics, piezoelectric materials, and ionic conductors. In all these materials, the physical properties are sensitively controlled by the oxygen stoichiometry. In extended crystal defects, e.g., grain boundaries and dislocations, the oxygen occupancy of lattice sites can deviate substantially from the stoichiometric value either because of structural constraints or as a result of the interaction of charged mobile oxygen vacancies with the electrically active defects.

As a starting point, it is necessary to understand the structure of defects and how their electronic properties are influenced by deviations of the local oxygen concentration from stoichiometry. A key issue in this endeavor is to be able to visualize the oxygen atoms as well as to measure the oxygen occupancy of lattice sites in or close to defects. Such measurements will allow us to construct the realistic structural models required to carry out quantum-mechanical calculations and to eventually control the electronic properties of real oxide materials.

Using an imaging mode based on the adjustment of a negative value of the spherical-aberration coefficient of the objective lens of a transmission electron microscope, we successfully imaged all types of atomic columns in the dielectric SrTiO3 and the superconductor YBa2Cu3O7. In particular, we were able to view the oxygen atoms which, due to their low scattering power, were not previously accessible, and this allowed us to detect local nonstoichiometries or the degree of oxygen-vacancy ordering. This technique offers interesting opportunities for research into oxides, minerals, and ceramics. In particular, this holds for the huge group of perovskite-derived electroceramic materials in which the local oxygen content sensitively controls the electronic properties.

Using high-resolution imaging at negative spherical aberration of the objective lens in an aberration-corrected transmission electron microscope, we measure the concentration of oxygen in Σ3{111} twin boundaries in BaTiO3 thin films at atomic resolution. On average, 68% of the boundary oxygen sites are occupied, and the others are left vacant. The modified Ti2O9 group unit thus formed reduces the grain boundary energy and provides a way of accommodating oxygen vacancies occurring in oxygen-deficient material by the formation of a nanotwin lamellae structure. The atomically resolved measurement technique offers the potential for studies on oxide materials in which the electronic properties sensitively depend on the local oxygen content.

Contact:

Dr. Chun-Lin Jia
Phone: +49 2461 61-2408
E-Mail: c.jia@fz-juelich.de