Atomic Resolution Study of the Interfacial Bonding at Si$_3$N$_4$/CeO$_{2-\delta}$ Grain Boundaries*

Using a combination of atomic resolution Z-contrast imaging and electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope, we examine the atomic and electronic structures at the interface between Si$_3$N$_4$ $(10 \overline{1}0)$ and CeO$_{2-\delta}$ inter-granular film (IGF). Ce atoms are observed to segregate to the interface in a two-layer periodic arrangement, which is significantly different compared to the structure observed in a previous study. Our EELS experiments show that {\bf (i)} oxygen is present at the interface in direct contact with the terminating Si$_3$N$_4$ open-ring structures, {\bf (ii)} the Ce valence state changes from +3 to +4 in going from the interface into the IGF, and {\bf (iii)} while the N concentration decreases away from the Si$_3$N$_4$ grains into the IGF, the Si concentration remains uniform across the whole width of the IGF. Possible reasons for these observed structural and electronic variations at the interface and their implications for future studies on Si$_3$N$_4$/rare-earth oxide interfaces are briefly discussed.