Nucleation of Solid-State Reactions between Nickel Oxide and Aluminium Oxide

The initial stages of the solid-state reaction between high-quality thin films of nickel oxide and single-crystal aluminum oxide have been investigated. Thin films were grown on specially cleaned and annealed basal-plane (0001) alumina substrates by pulsed-laser deposition. The thin-film reaction couples were then heat-treated in air at 1080° and 1100°C for different times in order to induce the formation of the nickel aluminate spinel. The reaction couples were characterized in cross section by high-resolution scanning electron microscopy and conventional transmission electron microscopy. The films of nickel oxide consisted of two twin variants with a common {111} plane, parallel to the basal plane of alumina, but rotated by 60° or 180° about the 〈111〉 direction normal to that plane. The spinel nucleated where the twin boundaries in the nickel oxide film met the alumina substrate and contained the same twin variants as the original NiO film.