Atomic-resolution observations of semicrystalline intergranular thin films in silicon nitride

Nanoscale intergranular films in doped silicon-nitride ceramics are known to markedly affect toughness and creep resistance. They are regarded as being fully amorphous, but are shown here to have a semicrystalline structure in a Ce-doped Si3N4. Using two different but complementary high-resolution electron-microscopy methods, the intergranular atomic structure, imaged with sub-angstrom resolution, reveals that segregated cerium ions take very periodic positions, along the intergranular-phase∕matrix-grain interface and as a semicrystalline structure spanning the width of the intergranular phase. This result has broad implications for the understanding of the structure and role of the intergranular phase in enhancing the mechanical properties of ceramics.