Crafting Spin-State Switchable Strain Profiles within RbxCo[Fe(CN)6]y@KjNi[Cr(CN)6]k Heterostructures

Spin-transition heterostructures have shown promise for inducing large switchable stresses at the nanoscale with a volumetric work density similar to piezoelectrics, but before practical applications are feasible, how heterostructure interfaces and geometry influence the transmission of stress and, in return, how they affect the spin-transition actuator itself, must be better understood. Here, four series of cubic spin-transition Prussian blue analogue (PBA) core–shell heterostructures were developed in order to probe the scaling behavior of the strain induced in the shell by the spin transition of the core. Cubic RbxCo[Fe(CN)6]y·nH2O (RbCoFe-PBA) particles ranging 100–600 nm were used to prepare separate series of RbxCo[Fe(CN)6]y·nH2O@KjNi[Cr(CN)6]k·mH2O (RbCoFe@KNiCr-PBA) core–shell particles with magnetic KNiCr-PBA shells ranging from 15 to 130 nm. A model fit to the strain-modified magnetization extracts the "strained volume" of the shell, and the results are compared with structural changes observed with powder X-ray diffraction. A linear relationship is found between the strained volume of the shell and the volume of the core for thicker shells, where the magnetic KNiCr-PBA shell is influenced to depths greater than 100 nm in response to the spin transition of the RbCoFe-PBA core. For thin shells, the relationship is more complicated, as the volume change in the actuating core and the strain it induces in the shell become interdependent and a function of shell thickness.