79. Structural and Magnetic Implications of Transition Metal Migration within Octahedral Core-Shell Nanocrystals
Beatriz Rivas-Murias, Martín Testa-Anta, Pau Torruella, Sònia Estradé, Francesca Peiró, Benito Rogríguez-González, Miguel Comesaña-Hermo and Verónica Salgueiriño
Chemistry of Materials (2020), 32, 10435-10446 (DOI: 10.1021/acs.chemmater.0c03017).
Octahedron-shaped cobalt oxide nanocrystals undergo a structural evolution once coated with thin shells of manganese or cobalt ferrite, by means of an asymmetric solid-solid diffusion occurring at the interface established between the oxides. The resultant mixed ferrites in the final nanostructures stem from the phase progression associated with a nonequilibrium kinetic product that evolves to reach the thermodynamic equilibrium. In this process, the initially strained crystalline lattice closer to the interface influences the progressive redistribution of Co2+ catoins diffusing out of the initial cobalt oxide core, dictating the final magnetic properties. When starting with a nonstoichiometric manganese ferrite shell, the preferential occupation of tetrahedral sites by Mn2+ cations forces the Co2+ to occupy octahedral sites, offering a Mn- and Co-doped magnetite shell onto the CoO core. However, when starting with a cobalt ferrite shell, an extra doping of Co2+ cations in the strained layers close to the interface forces this ferrite to transition from the inverse to the normal spienl structure, leading to core-shell nanocrystlas of CoO and Co-rich cobalt ferrite with an enhanced magnetic moment.