The determination of the interface structure between ionocovalent compounds: the general case study of the Al2O3/ZrO2 large mis-fit system

Abstract

Heterophase interfaces between alumina and zirconia are investigated by an HREM approach. The studied samples are synthesized by the sol–gel route, which allows the growth of thin islands of pure zirconia showing heteroepitaxial growth on (110)_sa sapphire substrates. Their heteroepitaxial relationship is first established by electron diffraction. Specific cross sectional samples are then prepared, normal to the in-plane common directions. Then, HREM images allow definition of the metric of the coincidence super-cell between the two lattices. The crystal chemistry of these heteroepitaxial interfaces is then presented for two peculiar orientation variants. The geometry of the interfacial polyhedra of coordination is presented and a 3D interface model is established for the main orientation. It is shown that the orientation relationship is governed by the occurrence of continuous cationic lattice planes across the interface. They correspond to ‘structural walls’ which are typical structural features of the orientation variants.

Alumina/zirconia represents a case of large mis-fit system. However, except for the expected geometrical dislocations and ledges, the interfaces present very few physical defects. However, the overall interfacial polyhedra of coordination suffer high distortion. The lattice mismatch could thus be locally accommodated by an important elastic strain linked to fluctuations of the cation–anion interatomic distances. The predominance of this relaxation mechanism, based on the adaptability of the ionocovalent bonds, seems a characteristic of oxide/oxide interfaces.