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

Abstract

Thin films of lanthanum hexaaluminate were produced on the basal plane of sapphire by chemical solution deposition and thermal treatment. The close-packed oxygen sublattices of the lanthanum hexaaluminate films with the magnetoplumbite structure and the sapphire are in exact topotactic alignment. The 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 object plane. Simulated electron waves based on different interface models were compared with the reconstructed waves, and very good matches of the phase images were obtained. Two different types of interface were observed and were characterized in detail along [1100]_HA-[21 10]_s (where the subscript 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 unrelaxed interfaces were derived. The type I interface is located at a common 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 plane of the hexaaluminate and an oxygen plane of the sapphire. It was found that the structural elements at the two interfaces agree with the structural features of known aluminium oxides, which is realized by an appropriate stacking seauence of the oxygen sublattices at the interface and the non-occupation of unfavoured sites.