Unveiling core-shell structure formation in a Ni3Fe nanocatalyst with in situ multi-Bragg coherent diffraction imaging.

Solid state reactions play a key role in materials science. The evolution of the structure of a single 350 nm Ni3Fe nanoparticle, i.e. its morphology (facets) as well as its deformation field, have been followed by applying multi-reflection Bragg coherent diffraction imaging. Through this approach, we unveiled a demixing process that occurs at high temperatures (600°C). This process leads to the gradual emergence of a highly strained core-shell structure, distinguished by two distinct lattice parameters with a di erence of 0.4%. Concurrently, this transformation causes the facets to vanish, ultimately yielding a rounded core-shell nanoparticle. This final structure comprises a Ni3Fe core surrounded by a 40 nm Ni-rich outer shell. Providing an in situ 3D imaging of the lattice parameters at the nanometer scale while varying the temperature, this study – with the support of atomistic simulations – not only showcases the power of in situ multi-reflection BCDI but also provide valuable insights into the mechanisms at work during a solid state reaction characterized by a core-shell transition.