Evidence for Interface-Induced Strain and Its Influence on Photomagnetism in Prussian Blue Analogue Core–Shell Heterostructures, Rb_aCo_b[Fe(CN)₆]_c·mH₂O@K_jNi_k[Cr(CN)₆]_l·nH₂O

Abstract

A series of photomagnetic coordination polymer core–shell heterostructures, based on the light-switchable Prussian blue analogue Rb a Co b [Fe­(CN)6] c ·mH2O (RbCoFe-PBA) as the core and the ferromagnetic K j Ni k [Cr­(CN)6] l ·nH2O (KNiCr-PBA) as the shell, was studied using powder X-ray diffraction, down to 100 K, and magnetometry, down to 2 K, to investigate the influence of the shell thickness on light-induced magnetization changes and gain insight into the mechanism. The core material is known to undergo a charge-transfer-induced spin transition (CTIST), and synchrotron powder diffraction was used to monitor structural changes in both the core and the shell associated with the thermally and optically induced CTIST of the core. Significant lattice contraction in the RbCoFe-PBA core upon cooling through the high-spin to the low-spin state transition near ∼260 K induces strain on the KNiCr-PBA shells. This lattice strain in the shell can be relieved either by thermal cycling back to high temperature or by using light to access the metastable high-spin state of the core at low temperature. The different extents of strain in the KNiCr-PBA shell are reflected in low-temperature, low-field magnetization versus temperature data in the light and dark states. A broader magnetic transition at T c ≈ 70 K in the dark state relative to the light state reflects the greater dispersion of nearest-neighbor contacts and exchange energies induced by the structural distortions of the strained state. Analyses for different shell thicknesses, coupled with high-field magnetization data, support a mechanism whereby the light-induced magnetization changes in the KNiCr-PBA shell are due to realignment of the local magnetic anisotropy as a result of the structural changes in the shell associated with the optical CTIST of the core. Through magnetization and structural analyses, the depth to which the properties of the shell are influenced by the core–shell architecture was estimated to be between 40 and 50 nm.

Elasticity of Prussian‐Blue‐Analogue Nanoparticles

“…Access to the elastic moduli of PBA nanoparticles, which are largely unknown in molecule-based materials (bulk and nano-object), is of paramount importance for developing applications based on the mechanical properties of molecular materials, such as microelectromechanical systems (MEMS) [36,37] or mechanically coupled nano-heterostructures. [22,24,38] It is important to stress also that in this work, we provide an accurate value of Poisson's ratio. The mechanical characterization of coordination networks is challenging, especially at the nanoscale, but it is necessary for the development of new applications and for understanding phase-change phenomena governed by electron-lattice coupling.…”

Spin Crossover Nanomaterials: From Fundamental Concepts to Devices

“…This latter is related in most cases to a charge transfer between the cyanide‐bridged metallic centers, denoted charge transfer induced spin transition (CTIST), which can occur due to thermal, pressure, or light stimuli . Recently, the groups of Mallah, Talham, and Maurin succeeded in synthesizing fairly monodisperse core–shell PBA@PBA nanoparticles with a CoFe core exhibiting CTIST and an epitaxial ferromagnetic shell consisting of either NiCr or CoCr analogs. Their experiments revealed in each case remarkable couplings between the properties of the constituents.…”

“…Thanks to the high quality crystalline samples a deep investigation of the underlying structural aspects became possible using high resolution X‐ray diffraction (XRD) and transmission electron microscopy (TEM) methods. As a representative example, Figure a shows a particle of the CTIST compound Rb 0.19 Co[Fe(CN) 6 ] 0.73 (1.4H 2 O)·1.8H 2 O embedded in a shell of the ferromagnetic K 0.28 Ni[Cr(CN) 6 ] 0.76 (1.4H 2 O)· n H 2 O . This structure exhibits photo‐induced and thermo‐induced changes in the magnetization of the KNiCr component, which is not light‐switchable on its own.…”

“…Upon cooling from 300 to 100 K the diffraction peaks of RbCoFe exhibit a spectacular shift to higher 2θ angles, which is also accompanied by a significant broadening of the peaks attributed to KNiCr. These effects can be inversed either by heating the particles back to 300 K or by light irradiation at 100 K. Reproduced with permission . Copyright 2016, American Chemical Society.…”

Prussian Blue Analogue Mesoframes for Enhanced Aqueous Sodium-ion Storage

“…By downsizing of a bulk crystal, the volume ratio of the surface units to the bulk units can be increased significantly, leading to a coupling effect from the bulk structure and the surface units [17,20,21]. For instance, the meso-sized coordination polymers can accommodate guest molecules still, but their phase-change type would be changed owing to the suppressing effect of the surface energy [14,[22][23][24]. Downsizing can also shorten the diffusion distance when the guest molecules/ions travel inside, leading to high-rate adsorption/desorption ability.…”