Microstructural Evolution of a Cu and θ-Al2O3 Composite Formed By Reduction of Delafossite CuAlO2: A HAADF-STEM Study

In situ reduction of bulk, polycrystalline copper(I) aluminate (CuAlO2) results in the formation of an intimate two-phase mixture of metallic Cu and theta-alumina. The microstructure of a partially transformed region was studied at the atomistic scale using high-angle angular-dark-field scanning transmission electron microscopy (HAADF-STEM). The observations were consistent with a topotactic transformation mechanism whereby deintercalation of the Cu atoms occurs sequentially at the edges of the Cu+ atomic layers of the CuAlO2 delafossite structure. The Cu forms faceted nanoislands that exhibit an orientation relationship with the theta-alumina matrix. There is also concomitant outward diffusion of oxygen, and it is suggested that the theta-alumina is formed by the consolidation of the layers of Al-O octahedra of the delafossite structure, with some local rearrangement of the Al3+ ions. This model is supported by the observed continuity of the Al-O layers between the parent CuAlO2 and theta-alumina, together with the orientation relationship (0003)CuAlO2// (40 (2) over bar)theta-alumina.