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 (CuAlO 2 ) results in the formation of an intimate two-phase mixture of metallic Cu and θ-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 CuAlO 2 delafossite structure. The Cu forms faceted nanoislands that exhibit an orientation relationship with the θ-alumina matrix. There is also concomitant outward diffusion of oxygen, and it is suggested that the θ-alumina is formed by the consolidation of the layers of Al–O octahedra of the delafossite structure, with some local rearrangement of the Al 3+ ions. This model is supported by the observed continuity of the Al–O layers between the parent CuAlO 2 and θ-alumina, together with the orientation relationship (0003)CuAlO 2 // (402̅)θ-alumina .