Thin‐Film Reactions

A new method for studying the formation of nickel‐aluminate spinel during the reaction between nickel oxide and alumina has been developed in which a thin‐film substrate of alumina is exposed to nickel oxide at elevated temperatures. Single crystal or twinned particles of spinel then grow in either near‐ or exact‐topotactic alignment along the edges of the thin‐film substrate. The spinel also grows on and into the surface of the substrate, but the deviation from exact topotaxy then tends to be larger. The expected topotactic relationship between the spinel and the alumina substrate is such that the (0001) plane of the alumina is parallel or near parallel to the {111} planes of the spinel with 〈1100〉 parallel to 〈110〉. However, a second common orientation relationship is also found where a {1120} plane of the alumina is parallel or nearly parallel to a {111} plane of the spinel; 〈1100〉 is again parallel to 〈110〉. First‐order twin boundaries in the spinel particles which have grown on the edge of the thin alumina substrate are found to be common just as they are in bulk‐reacted samples. A change in the orientation of the spinel with respect to the alumina substrate as a result of reannealing has been directly observed. The morphology of the various types of spinel particles is discussed briefly.