Probing Charge-State Distribution at Grain Boundaries Varied with Dopant Concentration for Ceria Ceramics

Knowledge of dopant-concentration dependent grain-boundary conductivity is a prerequisite for designing sophisticated nanostructured electrolytes as oxide ceramics but is not yet fully understood for the lack of quantitative information on the charge-state distribution there. Here, Gd-doped ceria (GDC) with nano- and micrometer sized grains, referred to as nano- and micro-GDC, is studied as a function of dopant concentration by means of element-specific positron annihilation spectroscopy. It is demonstrated that positrons are able to probe the negatively charged spaces adjacent to the grain boundary core in addition to the electrons bound to Ce atoms in the bulk. The atomic concentration of negatively charged space is similar to 10(-7) both for micro- and nano-GDC and increases as the Gd dopant increases. In contrast to that, the size of negatively charged space shrinks with the dopant concentration due to a charge effect arising from the electrons associated with the Gd dopant. The volume occupancy of negatively charged space satisfactorily explains the grain boundary conductivity dependent on the dopant concentration as well as the grain size in terms of the space-charge theory.