  Ionic conductivity blocking at grain boundaries in polycrystalline
electrolytes is one of the main obstacles that need to be overcome in order to
improve the performance of solid state fuel cells and batteries. To this aim,
harnessing the physical properties of grain boundaries in ionic conducting
materials such as yttria stabilized zirconia (YSZ) down to the atomic scale
arises as a greatly important task. Here we present a structural and
compositional analysis of a single grain boundary in a 9 mol% yttria content
YSZ bicrystal by means of aberration corrected scanning transmission electron
microscopy. Our studies combine strain and compositional atomic resolution
analysis with density-functional-theory calculations in order to find a
preferential segregation of yttrium to the expansive atomic sites at the grain
boundary dislocation cores. These results address a crucial step towards the
understanding of the physical properties of grain boundaries down to atomic
dimensions.
