Direct observation of charge transfer in double-perovskite-like RbMn[Fe(CN)6]

BEG spin-1 model with random exchange magnetic interactions for spin-crossover solids

We have investigated magnetic phase diagrams of spin-crossover (SCO) solids throughout the Blume-Emery-Griffiths spin-1 model where the spin states ±1 and 0 are associated to high-spin state and low-spin states respectively. In the present work, the quadrupolar interaction,K, parameter depends linearly on temperature and accounts for the role of the lattice phonons in the elastic interactions between the SCO units. Magnetic interactions are randomly distributed in the system and are controlled by a factorγ=Jij/Ksuch that forγ = 0 (Jij=0), magnetic ordering is not expected. The crystal-field that acts on SCO sites depends both on the ligand-field strength and the degeneracy ratio between HS and LS states as in some previous works. The system is also under the effect of a random local magnetic fieldhiacting on each sitei. The model is solved using a homogeneous mean field theory. Our investigations reveal the occurrence of thermally-induced gradual, and first-order spin-transitions by varying the model parameters. At vicinity of first-order transition, various types of isothermal magnetic hysteresis loops are obtained and their corresponding coercive field and loop patterns are discussed as function of temperature.

Dimensional Transformation of Molecular Magnetic Materials

Two-dimensional (2D) magnetic layered materials have revolutionized size dependent magnetism to manipulate spin-based devices. However, it has been challenging to artificially create 2D magnetic materials from three-dimensional (3D) crystal structures with a variety of material groups. Here, we present the dimensionality manipulation via cation exchange of a 3D Prussian blue analogue [RbMnFe(CN)₆] toward a 2D magnetic sheet [(K,Rb)(V,Mn)(Cr,Fe)(CN)₆] with the magnetic ordering temperature rising from 12 to 330 K. Such a 2D magnetic sheet achieves crystalline V-Cr coordination in the Prussian blue lattice with pronounced anisotropy and stimuli responsiveness. The pressure dependent magnetic tunability of such 2D networks is predicted using first-principles calculations and demonstrated using the phase transitions of the hydrogel. This previously unobserved phenomenon of dimensional manipulation of a bulk crystal structure provides a rational strategy to expand the diversity and chemical compositions of 2D molecular magnetic material libraries.

Rational Direct Synthesis of RbMnFe Nanoparticles (RbMnFe = RbxMn[Fe(CN)6](2+x)/3·nH2O Prussian Blue Analogue)

In this paper, we report the chemical strategy followed to obtain, in a direct way, nanoparticles of the Rb_xMn[Fe(CN)₆]_(x+2)/3·nH₂O (RbMnFe) Prussian blue analogue with the aim of keeping the switching ability of this compound at the nanoscale. The switching properties come from a reversible electron transfer between the iron and manganese ions and depends on the rubidium content in the structure that has to be higher than 0.6. Despite the multifunctionality of this family of compounds and its interest in various applications, no systematic studies were performed to obtain well-defined nanoparticles. This paper relates to such an investigation. To draw relationship between size reduction, composition, and switching properties, a special attention was brought to the determination of the composition through elemental analysis and structure refinement of powder X-ray diffraction patterns together with infrared spectroscopy and elemental analysis. Several chemical parameters were explored to control both the size reduction and the composition following a direct synthetic approach. The results show that the smaller the particles, the lower the rubidium content. This observation might prevent the observation of switching properties on very small particles. Despite this antagonist effect, we achieved switchable particles of around 200 nm without any use of surfactant. Moreover, the size reduction is associated with the observation of the electron transfer down to 52% of rubidium in the nanoparticles against 64% in microparticles. This work is of particular interest in processing such nanoparticles into devices.

Out-of-equilibrium dynamics driven by photoinduced charge transfer in CsCoFe Prussian Blue Analogue nanocrystals

In this paper we study the out-of-equilibrium dynamics associated with photoinduced charge-transfer (CT) in cyanide-bridged Co-Fe Prussian blue analogue nanocrystals. In these coordination networks, the structural trapping of the photoinduced...

Electron transfer phase transition and oxidization process in NaxCo0.44Mn0.56[Fe(CN)6]0.90 (0.00 ≤ x ≤ 1.60)

We investigated the phase diagram of Na_xCo_0.44Mn_0.56[Fe(CN)₆]_0.90 in the entire Na concentration range of 0.00 ≤ x ≤ 1.60. We found that the compound shows an electron transfer (ET) phase transition in a wide x range of 0.19 ≤ x ≤ 1.38. The extended ET model well reproduces the variation of the [Fe²⁺(CN)₆]^4- and [Fe³⁺(CN)₆]^3- concentration at the phase transition. The ET phase transition reverses the oxidation process of the compound; the process is in the order of Co, Mn, and Fe (Fe, Mn, and Co) in the low (high) temperature phase.

Exploring Ultrafast Photoswitching Pathways in RbMnFe Prussian Blue Analogue

We study by femtosecond optical pump-probe spectroscopy the photoinduced charge transfer (CT) in the RbMnFe Prussian blue analogue. Previous studies evidenced the local nature of the photoinduced Mn^III Fe^II → Mn^II Fe^III process, occurring within less than 1 ps. Here we show experimentally that two photoswitching pathways exist, depending on the excitation pump wavelength, which is confirmed by band structure calculations. Photoexcitation of α spins corresponds to the Mn(d-d) band, which drives reverse Jahn-Teller distortion through the population of antibonding Mn-N orbitals, and induces CT within ≈190 fs. The process launches coherent lattice torsion during the self-trapping of the CT small-polaron. Photoexcitation of β spins drives intervalence Fe→Mn CT towards non-bonding states and results in a slower dynamic.

Second harmonic generation on chiral cyanido-bridged FeII-NbIV spin-crossover complexes

Incorporating chiral organic ligands into cyanido-bridged FeII-NbIV assemblies synthesized chiral spin-crossover complexes, FeII2[NbIV(CN)8](L)8·6H2O (L = R-, S-, or rac-1-(3-pyridyl)ethanol: R-FeNb, S-FeNb, or rac-FeNb). Rietveld analyses based on a racemic complex of rac-FeNb indicate that the chiral complexes have a cubic crystal structure in the I213 space group with a three-dimensional cyanido-bridged FeII-NbIV coordination network. All the complexes exhibit spin crossover between the high-spin (HS) and the low-spin (LS) FeII states without thermal hysteresis. Chiral complexes of R-FeNb and S-FeNb show second harmonic generation (SHG) due to their non-centrosymmetric structure. The I213 space group provides second-order susceptibility tensor elements of χxyz, χyzx, and χzxy, which contribute to SHG. The temperature-dependent second harmonic light intensity change is due to spin crossover between FeIIHS and FeIILS.

Photoswitchable Nonlinear-Optical Crystal Based on a Dysprosium-Iron Nitrosyl Metal Assembly

Nitrosyl metal complexes (M-NO), in which nitrosyl ligands are coordinated to transition-metal ions, have been studied from the viewpoints of physiological activity, catalytic activity, and photosensitivity. The structural flexibility and electric polarization of the nitrosyl ligand are attractive characteristics. Herein we show a photoswitchable nonlinear-optical (NLO) crystal based on a dysprosium-iron nitrosyl assembly. This crystal is composed of a one-dimensional chain structure in the polar Pna2₁ space group. Because of spontaneous electric polarization, it exhibits a NLO effect of second harmonic generation (SHG). The SHG signal reversibly changes by alternate irradiation with 473 and 804 nm laser lights. The observed photoreversible switching effect on SHG is caused by photoinduced linkage isomerization of the metal nitrosyl sites, i.e., M-N⁺═O ↔ M-O═N⁺. Such an optically switchable NLO crystal should be useful for optical devices such as optical filters and optical shutters as well as probes in SHG microscopy.

Neodymium β-diketonate showing slow magnetic relaxation and acting as a ratiometric thermometer based on near-infrared emission

Self-assembly of β-diketonate (Htta = thenoyl(trifluoro)acetone) and 4,4′-azopyridine (Azo-py) with neodymium(iii) ions in the presence of methanol resulted in the formation of mononuclear complex [Nd^III(TTA)₃(MeOH)₂]·0.5Azo-py (A) in which two asymmetric units are linked by Azo-py through hydrogen bonding via methanol. A reveals near-infrared emission (NIR) centred at about 895 and 1056 nm, in the 10–370 K temperature range, originating from the two emissive transitions on Nd(iii) from ⁴F_3/2 to ⁴I_9/2 and ⁴I_11/2 levels, respectively. Furthermore, the NIR luminescence intensity of A at room temperature augments two times upon thermal elimination of one coordinated methanol molecule. The thermally activated A exhibits single centre ratiometric thermometer behaviour in a wide temperature range from 10 to 300 K. Moreover, fluorescence properties of A were compared to another mononuclear complex [Nd^III(TTA)₃(4-OHpy)(H₂O)] (B). Assembly A also exhibits field-induced slow magnetic relaxation properties with an energy barrier of ΔE/k_B = 19.7(7) K and an attempt time of relaxation, τ₀ = 3.7(8) × 10⁻⁷ s for fresh sample A, and ΔE/k_B = 27.3 K and τ₀ = 8.5(0) × 10⁻⁸ for assembly A after thermal treatment at 370 K.

In this work, we investigated thermally-induced Nd-based one centred ratiometric thermometer which also exhibit single molecule magnetic properties.

Chiral cyanido-bridged Mn–Nb magnets including halogen-bonds

Chiral and achiral three-dimensional cyanido-bridged metal assemblies with 4-halopyridine, [Mn^II(4-Xpy)₄]₂[Nb^IV(CN)₈] (X = I, 1; X = Cl, 2), are prepared. 1 and 2 show ferrimagnetism with T_C of 22 and 28 K, respectively. Chiral compound 1 exhibits second-harmonic generation.

Distinguishing between High- and Low-Spin States for Divalent Mn in Mn-Based Prussian Blue Analogue by High-Resolution Soft X-ray Emission Spectroscopy

We combine Mn L2,3-edge X-ray absorption, high resolution Mn 2p-3d-2p resonant X-ray emission, and configuration-interaction full-multiplet (CIFM) calculation to analyze the electronic structure of Mn-based Prussian blue analogue. We clarified the Mn 3d energy diagram for the Mn(2+) low-spin state separately from that of the Mn(2+) high-spin state by tuning the excitation energy for the X-ray emission measurement. The obtained X-ray emission spectra are generally reproduced by the CIFM calculation for the Mn(2+) low spin state having a stronger ligand-to-metal charge-transfer effect between Mn t2g and CN π orbitals than the Mn(2+) high spin state. The d-d-excitation peak nearest to the elastic scattering was ascribed to the Mn(2+) LS state by the CIFM calculation, indicating that the Mn(2+) LS state with a hole on the t2g orbital locates near the Fermi level.

Contemporary X-ray electron-density studies using synchrotron radiation

Synchrotron radiation has many compelling advantages over conventional radiation sources in the measurement of accurate Bragg diffraction data. The variable photon energy and much higher flux may help to minimize critical systematic effects such as absorption, extinction and anomalous scattering. Based on a survey of selected published results from the last decade, the benefits of using synchrotron radiation in the determination of X-ray electron densities are discussed, and possible future directions of this field are examined.

Photomagnetism in cyano-bridged bimetal assemblies

The study of photoinduced phase-transition materials has implications for the fields of inorganic chemistry, solid-state chemistry, and materials science. Cyano-bridged bimetal assemblies are promising photomagnetic materials. Because cyano-bridged bimetal assemblies possess various absorption bands in the visible light region, their electronic and spin states can be controlled by visible light irradiation. Moreover, the selection of magnetic metal ions and organic ligands provide a way of controlling spin-spin interactions through a cyano bridge. In this Account, we describe cyano-bridged bimetal assemblies developed in our laboratory. Cu(II)(2)[Mo(IV)(CN)(8)]·8H(2)O (CuMo), Rb(I)Mn(II)[Fe(III)(CN)(6)] (RbMnFe), and Co(II)(3)[W(V)(CN)(8)](2)·(pyrimidine)(4)·6H(2)O (CoW) induce photomagnetism via photoinduced metal-to-metal charge transfers (MM'CT), while Fe(II)(2)[Nb(IV)(CN)(8)]·(4-pyridinealdoxime)(8)·2H(2)O (FeNb) exhibits a photoinduced magnetization via a photoinduced spin crossover. Irradiation with 473 nm light causes the CuMo system to exhibit a spontaneous magnetization with a Curie temperature (T(C)) of 25 K, but irradiation with 532, 785, and 840 nm light reduces the magnetization. In this reversible photomagnetic process, excitation of the MM'CT from Mo(IV) to Cu(II) produces a ferromagnetic mixed-valence isomer of Cu(I)Cu(II)[Mo(V)(CN)(8)]·8H(2)O (CuMo'). CuMo' returns to CuMo upon irradiation in the reverse-M'MCT band. RbMnFe shows a charge transfer (CT)-induced phase transition from the Mn(II)-Fe(III) phase to the Mn(III)-Fe(II) phase. Irradiation with 532 nm light converts the Mn(III)-Fe(II) phase into the Mn(II)-Fe(III) phase, and we observe photodemagnetization. In contrast, irradiation of the Mn(II)-Fe(III) phase with 410 nm light causes the reverse phase transition. A CT-induced Jahn-Teller distortion is responsible for this visible light-induced reversible photomagnetic effect. In the CoW system, a CT-induced spin transition causes the thermal phase transition from the Co(II)-W(V) phase to the Co(III)-W(IV) phase. Irradiation of the Co(III)-W(IV) phase with 840 nm light causes ferromagnetism with a T(C) of 40 K and magnetic coercive field (H(c)) of 12,000 Oe, but excitation of the back M'MCT (Co(II) → W(V)) with 532 nm light leads to the reverse phase transition. These examples of the photomagnetic effect have occurred by exciting MM'CT bands. In the fields of inorganic chemistry and materials science, researchers have studied extensively the photoinduced phase transitions between low-spin (LS) and high-spin (HS) transition metal ions. Recently, we have observed the first example of photoinduced spin crossover ferromagnetism with a FeNb system (T(C) = 20 K and H(c) = 240 Oe), in which a strong superexchange interaction between photoproduced Fe(II)(HS) and neighboring paramagnetic Nb(IV) operates through a CN bridge. The optical switching magnets described in this Account may lead to novel optical recording technologies such as optomagnetic memories and optical computers.

Structure and magnetic ordering of the anomalous layered (2D) ferrimagnet [NEt4]2Mn(II)3(CN)8 and 3D bridged-layered antiferromagnet [NEt4]Mn(II)3(CN)7 Prussian blue analogues

The reaction of Mn(II) and [NEt(4)]CN leads to the isolation of solvated [NEt(4)]Mn(3)(CN)(7) (1) and [NEt(4)](2) Mn(3)(CN)(8) (2), which have hexagonal unit cells [1: R3m, a = 8.0738(1), c = 29.086(1) Å; 2: P3m1, a = 7.9992(3), c = 14.014(1) Å] rather than the face centered cubic lattice that is typical of Prussian blue structured materials. The formula units of both 1 and 2 are composed of one low- and two high-spin Mn(II) ions. Each low-spin, octahedral [Mn(II)(CN)(6)](4-) bonds to six high-spin tetrahedral Mn(II) ions through the N atoms, and each of the tetrahedral Mn(II) ions are bound to three low-spin octahedral [Mn(II)(CN)(6)](4-) moieties. For 2, the fourth cyanide on the tetrahedral Mn(II) site is C bound and is terminal. In contrast, it is orientationally disordered and bridges two tetrahedral Mn(II) centers for 1 forming an extended 3D network structure. The layers of octahedra are separated by 14.01 Å (c axis) for 2, and 9.70 Å (c/3) for 1. The [NEt(4)](+) cations and solvent are disordered and reside between the layers. Both 1 and 2 possess antiferromagnetic superexchange coupling between each low-spin (S = 1/2) octahedral Mn(II) site and two high-spin (S = 5/2) tetrahedral Mn(II) sites within a layer. Analogue 2 orders as a ferrimagnet at 27(±1) K with a coercive field and remanent magnetization of 1140 Oe and 22,800 emuOe mol(-1), respectively, and the magnetization approaches saturation of 49,800 emuOe mol(-1) at 90,000 Oe. In contrast, the bonding via bridging cyanides between the ferrimagnetic layers leads to antiferromagnetic coupling, and 3D structured 1 has a different magnetic behavior to 2. Thus, 1 is a Prussian blue analogue with an antiferromagnetic ground state [T(c) = 27 K from d(χT)/dT].

Electric-field-induced charge-transfer phase transition: a promising approach toward electrically switchable devices

Much research has been directed toward the development of electrically switchable optical materials for applications in memory and display devices. Here we present experimental evidence for an electric-field-induced charge-transfer phase transition in two cyanometalate complexes: Rb(0.8)Mn[Fe(CN)(6)](0.93).1.62H(2)O and Co(3)[W(CN)(8)](2)(pyrimidine)(4).6H(2)O, involving changes in their magnetic, optical, and electronic properties as well. Application of an electric field above a threshold value and within the thermal hysteresis region leads to a transition from the high- to the low-temperature phase in these compounds. A model is proposed to explain the main observations on the basis of a para-ferroelectric transition. Our observations suggest that this new concept of electrical switching, based on materials exhibiting charge-transfer phase transitions with large thermal hysteresis loops, may open up doors for novel electro-optical devices.

Poly[[hexa-μ-cyanido-manganese(II)iron(III)] penta-hydrate]

The structure of the title compound, Mn^II[Fe^III(CN)₆]_2/3·5H₂O, features a face-centered cubic –Mn—NC—Fe– framework with both Mn and Fe having site symmetry mm. Since one-third of the [Fe(CN)₆]³⁻ units are missing for a given formula in order to maintain charge neutrality, each Mn atom around such a vacancy is coordinated not only by the N atoms of the CN groups but also by the O atoms of the ligand water mol­ecules. In addition to ligand water mol­ecules, two types of non-coordinated water mol­ecules, so-called zeolitic water mol­ecules, exist in the inter­stitial sites of the –Mn—NC—Fe– framework. The positions of the O atoms of the zeolitic water mol­ecules are fixed by the linkage via hydrogen bonds between ligand water and zeolitic water mol­ecules. The structure is related to a recently reported rubidium manganese hexa­cyano­ferrate. Site occupancy factors for Fe, C, N are 0.67; for two O atoms the value is 0.83 and for one O atom is 0.17.

The MEM/Rietveld method with nano-applications – accurate charge-density studies of nano-structured materials by synchrotron-radiation powder diffraction

Structural studies of materials with nano-sized spaces, called nano-structured materials, have been carried out by high-resolution powder diffraction. Our developed analytical method, which is the combination of the maximum-entropy method (MEM) and Rietveld refinement, the so-called MEM/Rietveld method, has been successfully applied to the analysis of synchrotron-radiation (SR) powder diffraction data measured at SPring-8, a third-generation SR light source. In this article, structural studies of nano-porous coordination polymers and endohedral metallofullerenes are presented with the advanced technique of SR powder experiment. The structure of the adsorbed guest molecule in the coordination polymer and encapsulated atoms in the fullerene cage are clearly revealed by the MEM charge density. The methodology of MEM/Rietveld analysis is also presented.

Control of Magnetic Properties through External Stimuli

The magnetic properties of many magnetic materials can be controlled by external stimuli. The principal focus here is on the thermal, photochemical, electrochemical, and chemical control of phase transitions that involve changes in magnetization. The molecular compounds described herein range from metal complexes, through pure organic compounds to composite materials. Most of the Review is devoted to the properties of valence-tautomeric compounds, molecular magnets, and spin-crossover complexes, which could find future application in memory devices or optical switches.

Nonlinear Magnetooptical Effects Caused by Piezoelectric Ferromagnetism in F4̄3m-type Prussian Blue Analogues

The second harmonic generation (SHG) and magnetization-induced SHG (MSHG) of AMA[MB(CN)6]-type (F3m) Prussian blue analogues (i.e., CsCo[Cr(CN)6].0.5H2O and RbMn[Fe(CN)6]) were observed. A large interaction between the nonlinear optical response and magnetic spins was observed in CsCo[Cr(CN)6].0.5H2O. These observations of SHG and MSHG imply that AMA[MB(CN)6]-type Prussian blue analogues are piezoelectric above the Curie temperature (TC) and piezoelectric ferromagnet below TC.