$\beta -$Si$_{3}$N$_{4}$/CeO$_{2-x }$Interface Investigated via Atomic Resolution Z-contrast Imaging, Electron Energy-Loss Spectroscopy and First-Principles Methods*

The addition of rare-earth oxides, typically forming intergranular glassy films in ceramics, has long been known to markedly affect toughness and creep resistance of Si$_{3}$N$_{4}$. The present work investigates the bonding characteristics of cerium at the interface between Si$_{3}$N$_{4 }$grains and the secondary ceria phases with aberration-corrected scanning transmission electron microscopy techniques. The obtained Z-contrast images and the electron energy-loss spectra taken at the interface of Si$_{3}$N$_{4}$/CeO$_{2-x}$ suggest that the arrangement of Ce at the interface depends on the thickness of the intergranular film, the electronic structure of the rare earth element, as well as the termination of Si$_{3}$N$_{4}$. Possible reasons for these observed structural and electronic variations at the interface, and their agreement with the theoretical predictions of two stoichiometric terminations of Si$_{3}$N$_{4}(10\mathop 1\limits^- 0)$ surface will be briefly discussed.