Interfaces between lanthanum hexaaluminate and sapphire studied by high-resolution electron microscopy

Thin films of lanthanum hexaaluminate were produced on the basal plane of s apphire by chemical solution deposition and thermal treatment. The close-pa cked oxygen sublattices of the lanthanum hexaaluminate films with the magne toplumbite structure and the sapphire are in exact topotactic alignment. Th e structure of the interfaces were investigated by high-resolution electron microscopy, and the images of focus series were used to invert the imaging process and finally to reconstruct the complex electron exit wave at the o bject plane. Simulated electron waves based on different interface models w ere compared with the reconstructed waves, and very good matches of the pha se images were obtained. Two different types of interface were observed and were characterized in detail along [1100](HA)-[2110](S) (where the subscri pt HA indicates the lanthanum hexaaluminate and the subscript S sapphire). Using criteria of structural chemistry, possible stacking sequences of the oxygen sublattices across the boundary were determined and two models of un relaxed interfaces were derived. The type I interface is located at a commo n oxygen plane of sapphire and a mixed layer of tetrahedral and octahedral cation sites of the spinel block of hexaaluminate. The type II interface is positioned at a partially occupied aluminium layer between the mirror plan e of the hexaaluminate and an oxygen plane of the sapphire. It was found th at the structural elements at the two interfaces agree with the structural features of known aluminium oxides, which is realized by an appropriate sta cking sequence of the oxygen sublattices at the interface and the non-occup ation of unfavoured sites.