Structure and chemistry of interfaces between ceria and yttria-stabilized zirconia studied by analytical STEM.

Epitaxial undoped and GdO-doped ceria films were grown by pulsed laser deposition on (1 1 1) faced YO-stabilized zirconia (YSZ). Highly localized cerium reduction at the film-substrate interfaces is revealed by atomically resolved valence EELS mapping using C aberration-corrected scanning transmission electron microscopy. The chemical profiles reveal interdiffusion of Ce, (Gd), Y, Zr, forming an intermixing zone at the interface 7-9 (1 1 1) lattice planes wide. In its vicinity, the fraction of Ce raises gradually over 6-8 lattice planes from zero in the bulk ceria to ≈100% in one single plane at the interface. Beyond this plane the Ce fraction drops sharply within the YSZ substrate. In the vicinity of the interface systematic scan deflections are observed during EELS line scans. The advancing electron probe experiences a retarding force at the ceria side, and an accelerating force at the YSZ side, irrespective of the scan direction. This behavior is suggestive of coulombic interactions between the electron probe and a charged interface. This is interpreted as an indication of the presence of a space-charge situation at the YSZ/ceria interface, resulting from an excess negative charge at the ceria side (due to Cecations) and an excess positive charge at the YSZ side (due to oxygen vacancies).