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Register a new userHidden Diversity of Vacancy Networks in Prussian Blue Analogues
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Prussian blue analogues (PBAs) are a broad and important family of microporous inorganic solids, famous for their gas storage, metal-ion immobilisation, proton conduction, and stimuli-dependent magnetic, electronic and optical properties. The family also includes the widely-used double-metal cyanide (DMC) catalysts and the topical hexacyanoferrate/hexacyanomanganate (HCF/HCM) battery materials. Central to the various physical properties of PBAs is the ability to transport mass reversibly, a process made possible by structural vacancies. Normally presumed random, vacancy arrangements are actually crucially important because they control the connectivity of the micropore network, and hence diffusivity and adsorption profiles. The long-standing obstacle to characterising PBA vacancy networks has always been the relative inaccessibility of single-crystal samples. Here we report the growth of single crystals of a range of PBAs. By measuring and interpreting their X-ray diffuse scattering patterns, we identify for the first time a striking diversity of non-random vacancy arrangements that is hidden from conventional crystallographic analysis of powder samples. Moreover, we show that this unexpected phase complexity can be understood in terms of a remarkably simple microscopic model based on local rules of electroneutrality and centrosymmetry. The hidden phase boundaries that emerge demarcate vacancy-network polymorphs with profoundly different micropore characteristics. Our results establish a clear foundation for correlated defect engineering in PBAs as a means of controlling storage capacity, anisotropy, and transport efficiency.
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Many Prussian Blue Analogues are known to show a thermally induced phase transition close to room temperature and a reversible, photo-induced phase transition at low temperatures. This work reports on magnetic measurements, X-ray photoemission and Raman spectroscopy on a particular class of these molecular heterobimetallic systems, specifically on Rb0.81Mn[Fe(CN)6]0.95_1.24H2O, Rb0.97Mn[Fe(CN)6]0.98_1.03H2O and Rb0.70Cu0.22Mn0.78[Fe(CN)6]0.86_2.05H2O, to investigate these transition phenomena both in the bulk of the material and at the sample surface. Results indicate a high degree of charge transfer in the bulk, while a substantially reduced conversion is found at the sample surface, even in case of a near perfect (Rb:Mn:Fe=1:1:1) stoichiometry. Thus, the intrinsic incompleteness of the charge transfer transition in these materials is found to be primarily due to surface reconstruction. Substitution of a large fraction of charge transfer active Mn ions by charge transfer inactive Cu ions leads to a proportional conversion reduction with respect to the maximum conversion that is still stoichiometrically possible and shows the charge transfer capability of metal centers to be quite robust upon inclusion of a neighboring impurity. Additionally, a 532 nm photo-induced metastable state, reminiscent of the high temperature Fe(III)Mn(II) ground state, is found at temperatures 50-100 K. The efficiency of photo-excitation to the metastable state is found to be maximized around 90 K. The photo-induced state is observed to relax to the low temperature Fe(II)Mn(III) ground state at a temperature of approximately 123 K.
Cubic heterostructured (BA) particles of Prussian blue analogues, composed of a shell of ferromagnetic K_{0.3}Ni[Cr(CN)_6]_{0.8} cdot 1.3H_2O (A), Tc ~ 70 K, surrounding a bulk core of photoactive ferrimagnetic Rb_{0.4}Co[Fe(CN)_6]_{0.8} cdot 1.2H_2O (B), Tc ~20 K, have been studied. Below Tc ~ 70 K, these samples exhibit a persistent photoinduced decrease in low-field magnetization, and these results resemble data from other core-shell particles and analogous ABA heterostructured films. This net decrease suggests that the photoinduced lattice expansion in the B layer generates a strain-induced decrease in the magnetization of the A layer, similar to a pressure-induced decrease observed by others in a pure A-like material and by us in our BA cubes. Upon further examination, the data also reveal a significant portion of the A material whose superexchange, J, is perturbed by the photoinduced strain from the B constituent.
The magnetic anisotropy of thin (~ 200 nm) and thick (~ 2 $mu$m) films and of polycrystalline (diameters ~ 60 nm) powders of the Prussian blue analogue Rb$_{0.7}$Ni$_{4.0}$[Cr(CN)$_6$]$_{2.9} cdot n$H$_2$O, a ferromagnetic material with $T_c sim 70$ K, have been investigated by magnetization, ESR at 50 GHz and 116 GHz, and variable-temperature x-ray diffraction (XRD). The origin of the anisotropic magnetic response cannot be attributed to the direct influence of the solid support, but the film growth protocol that preserves an organized two-dimensional film is important. In addition, the anisotropy does not arise from an anisotropic g-tensor nor from magneto-lattice variations above and below $T_c$. By considering effects due to magnetic domains and demagnetization factors, the analysis provides reasonable descriptions of the low and high field data, thereby identifying the origin of the magnetic anisotropy.
This paper summarizes 0 GPa to 0.6 GPa neutron diffraction measurements of a nickel hexacyanochromate coordination polymer (NiCrPB) that has the face-centered cubic, Prussian blue structure. Deuterated powders of NiCrPB contain ~100 nm sided cubic particles. The application of a large magnetic field shows the ambient pressure, saturated magnetic structure. Pressures of less than 1 GPa have previously been shown to decrease the magnetic susceptibility by as much as half, and we find modifications to the nuclear crystal structure at these pressures that we quantify. Bridging cyanide molecules isomerize their coordination direction under pressure to change the local ligand field and introduce inhomogeneities in the local (magnetic) anisotropy that act as pinning sites for magnetic domains, thereby reducing the low field magnetic susceptibility.
Magnetic order in the thermally quenched photomagnetic Prussian blue analogue coordination polymer K0.27Co[Fe(CN)6]0.73[D2O6]0.27 1.42D2O has been studied down to 4 K with unpolarized and polarized neutron powder diffraction as a function of applied magnetic field. Analysis of the data allows the onsite coherent magnetization of the Co and Fe spins to be established. Specifically, magnetic fields of 1 T and 4 T induce moments parallel to the applied field, and the sample behaves as a ferromagnet with a wandering axis.
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