We have determined the structures of two phases of unsolvated Mg(BH(4))(2), a material of interest for hydrogen storage. One or both phases can be obtained depending on the synthesis conditions. The first, a hexagonal phase with space group P6(1), is stable below 453 K. Upon heating above that temperature it transforms to an orthorhombic phase, with space group Fddd, stable to 613 K at which point it decomposes with hydrogen release. Both phases consist of complex networks of corner-sharing tetrahedra consisting of a central Mg atom and four BH(4) units. The high-temperature orthorhombic phase has a strong antisite disorder in the a lattice direction, which can be understood on the basis of atomic structure.

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Heterostructured ABA thin films consisting of two different Prussian blue analogues, where A is a ferromagnet and B is a photoinducible ferrimagnet, have been fabricated for the first time. This novel arrangement allows the magnetization to be decreased by irradiation with white light and significantly increases the ordering temperature of the photoinduced magnetism from 18 to 75 K.

Particles of formula Rb0.24Co[Fe(CN)6]0.74@K0.10Co[Cr(CN)6]0.70·nH2O with a light-responsive rubidium cobalt hexacyanoferrate (RbCoFe) core and a magnetic potassium cobalt hexacyanochromate (KCoCr) shell have been prepared and exhibit light-induced changes in the magnetization of the normally light-insensitive KCoCr shell, a new property resulting from the synergy between the core and shell of a coordination polymer heterostructure. A single batch of 135 ± 12 nm RbCoFe particles are used as seeds to generate three different core@shell samples, with KCoCr shell thicknesses of approximately 11, 23 and 37 nm, to probe the influence of the shell thickness over the particles' morphology and structural and magnetic properties. Synchrotron powder X-ray diffraction reveals that structural changes in the shell accompany the charge transfer induced spin transition (CTIST) of the core, giving direct evidence that the photomagnetic response of the shell is magnetomechanical in origin. The depth to which the KCoCr shell contributes to changes in magnetization is estimated to be approximately 24 nm when using a model that assumes a constant magnetic response of the core within the series of particles. In turn, the presence of the shell changes the nature of the CTIST of the core. As opposed to the usually observed first order transition exhibiting hysteresis, the CTIST becomes continuous in the core@shell particles.

Heterostructured thin films consisting of distinct layers of the Prussian blue analogues Rb a Co b [Fe(CN)6] c ·mH2O (CoFe PBA) and Rb j M k [Cr(CN)6] l ·nH2O (MCr PBA, where M = Ni or Co) have been fabricated, and their photomagnetic properties have been investigated. The CoFe PBA is known to be photoactive, with light induced changes in the unit cell size and the spin states below ∼150 K and magnetic order below ∼20 K. The NiCr and CoCr PBAs do not have native photoeffects, but are known to have higher magnetic ordering temperatures (T C NiCr ∼ 70 K, T C CoCr ∼ 30 K), and a pressure dependence of the magnetization. The layered heterostructures are synthesized using aqueous chemistry and sequential adsorption techniques that allow for fine control of layer thickness. Some of the heterostructured films show photoinduced magnetization changes up to the ordering temperatures of the MCr PBA component, behavior that is not seen when the individual materials are measured separately. A variety of different layer arrangements and thicknesses has been investigated with the goal of identifying structures that optimize the photocontrol of the magnetic response in the MCr PBA lattices, which are in intimate contact with the photoactive CoFe PBA lattices. The new behavior is optimized when the constituent layers have thicknesses on the order of hundreds of nanometers. When layers are too thin, it is shown that mixing of ions at the interface between PBA components leads to mixed-metal phases. The concurrence of the maximum temperature of the large photomagnetic effect with the native ordering temperature of the MCr PBA lattice, as well as its magnetic field dependence, supports the interpretation that the photocontrol is the result of photoinduced structural changes in the CoFe PBA lattice coupling to the MCr PBA component of the heterostructure, inducing random magnetic anisotropy.

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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.

The rate of the light-induced spin transition in a coordination polymer network solid dramatically increases when included as the core in mesoscale core-shell particles. A series of photomagnetic coordination polymer core-shell heterostructures, based on the light-switchable Rb Co[Fe(CN)] · mHO (RbCoFe-PBA) as core with the isostructural K Ni[Cr(CN)] · nHO (KNiCr-PBA) as shell, are studied using temperature-dependent powder X-ray diffraction and SQUID magnetometry. The core RbCoFe-PBA exhibits a charge transfer-induced spin transition (CTIST), which can be thermally and optically induced. When coupled to the shell, the rate of the optically induced transition from low spin to high spin increases. Isothermal relaxation from the optically induced high spin state of the core back to the low spin state and activation energies associated with the transition between these states were measured. The presence of a shell decreases the activation energy, which is associated with the elastic properties of the core. Numerical simulations using an electro-elastic model for the spin transition in core-shell particles supports the findings, demonstrating how coupling of the core to the shell changes the elastic properties of the system. The ability to tune the rate of optically induced magnetic and structural phase transitions through control of mesoscale architecture presents a new approach to the development of photoswitchable materials with tailored properties.