Photostrictive/Piezomagnetic
Core–Shell Particles Based on Prussian Blue Analogues: Evidence
for Confinement Effects?
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Abstract
High-quality core–shell particles,
which associate a photostrictive core (Rb<sub>0.5</sub>Co[Fe(CN)<sub>6</sub>]<sub>0.8</sub>·<i>z</i>H<sub>2</sub>O, <b>RbCoFe</b>) and a ferromagnetic shell (Rb<sub>0.2</sub>Ni[Cr(CN)<sub>6</sub>]<sub>0.7</sub>·<i>z</i>′H<sub>2</sub>O, <b>RbNiCr</b>), were successfully grown by a multistep protocol
based on coprecipitation in water. High-resolution transmission electron
microscopy shows that well-defined heterostructures are formed and
that the core–shell interface is abrupt with the epitaxial
relationship [001](001)<b>RbCoFe</b>//[001](001)<b>RbNiCr</b>, confirmed by simulations of the X-ray diffraction line widths.
The core particles are monocrystalline, with 50 nm sides, and the
shell consists of large platelet-like crystallites, with a height
that corresponds to the shell thickness and lateral dimensions comparable
to the size of the core particles. Analysis of the diffracted intensities
as a function of shell thickness (9–26 nm) shows that the epitaxial
shell growth does not lead to a thick pseudomorphic layer at the interface.
In contrast, Williamson–Hall plots suggest that a structural
relaxation takes place to adapt the mismatched lattices, with the
formation of misfit dislocations distributed over the entire shell
thickness. This later finding is indicative of an effective mechanical
coupling within the heterostructures. However, a magnetization increase
by only a few percent was observed under light irradiation for these <b>RbCoFe</b>@<b>RbNiCr</b> particles. We showed from in situ
synchrotron X-ray diffraction measurements that these small changes
most likely reflect confinement effects as photoswitching of the core
phase is partly or completely blocked depending on the shell thickness
