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

The charge density distribution has been determined for a transition metal cyanide, RbMn[Fe(CN)(6)], by means of the maximum entropy-Rietveld method combined with the highly angularly resolved synchrotron radiation x-ray powder diffraction at SPring-8 BL02B2. We directly observed a charge transfer from the Mn site to the Fe site in the low-temperature phase. On the basis of a local density approximation calculation, we discuss the origin for the anisotropic bonding electron distribution around the Mn3+ ion in the low-temperature phase.     

Incommensurate Phases of the MgSiF6·6H2O Crystals: EPR and Group-Theoretical Studies

The group-theoretical study of the structural phase transition to incommensurate state of MgSiF₆·6H₂O crystals, revealed by the electron paramagnetic resonance (EPR) method, as well as analysis of the EPR results, are presented. The consideration of temperature dependences of Mn²⁺ admixture ion EPR spectrum symmetry and parameters leads to the conclusion that at T _i1 = 370 ± 0.3 K they undergo second-order structural phase transition to incommensurately modulated state, the order parameter of this transition may be the angle of [Mg(H₂O)₆]²⁺ octahedra rotation around crystal C ₃ axis. At temperature decreasing below T _i1 the gradual transformation of plane-wave modulation of lattice displacements into soliton mode occurs, which is interrupted by the first-order phase transition at T _i2 = 343 ± 0.3 K accompanied by abrupt decrease in modulation amplitude. At T _c = 298.5 ± 0.3 K the first-order improper ferroelastic phase transition into monoclinic phase occurs. The group-theoretical analysis of the phase transition at T _i1 in the investigated crystals, carried out for the first time, has shown that the existence of the incommensurately modulated phase is conditioned by the fundamental reasons (presence of Lifshitz invariant). The conclusions of this analysis on the nature of order parameter, the structural motifs of incommensurate phase and the possible character of temperature evolution of the structure are in agreement with the EPR investigation data.     

Magnetic and Mössbauer studies of cyanide-bridged bimetallic assembly [Mn(cyclam)][Fe(CN)6]·3H2O

Cyanide-bridged bimetallic assembly [Mn(cyclam)][Fe(CN)₆]·3H₂O (cyclam=1,4,8,11-tetraazacyclotetradecane) was synthesized from the reaction of trans-[MnCl₂(cyclam)]Cl with K₃Fe(CN)₆. A linear chain structure consisting of alternating [Mn(cyclam)]³⁺ and [Fe(CN)₆]³⁻ units was indicated by the IR and Mössbauer spectra. The variable-temperature magnetization and Mössbauer measurements revealed that this complex exhibited a long-range ordering below 6.8 K. The magnetic behavior of the complex was based on intrachain ferromagnetic and interchain antiferromagnetic interactions.     

Mössbauer studies of photochemical reaction products of Ru(bpy)3 2+ iron cyanide double complexes

The reductive and the oxidative electron-transfer photochemical reaction system of light-irradiated the mix solutions of Ru(bpy)₃ ²⁺ with [Fe(CN)₆]^4–, [Fe(CN)₆]^3–, [Fe(CN)₅NO]^2– and PB (Prussian Blue) have been studied. The double complexes which isolated from the precipitates of the photochemical reaction have been identified by means of Mössbauer spectroscopy. In order to clarify the chemical states of these isolated double complexes, we have (prepared and) studied Mössbauer spectra of the double complexes such as [Ru(bpy)₃]₃[Fe(CN)₆]₂.14H₂O, [Ru(bpy)₃]₂[Fe(CN)₆].10H₂O, [Ru(bpy)₃][Fe(CN)₅NO].4H₂O, and [Ru(bpy)₃][PB]₂.xH₂O.     

Photo-induced phase transitions probed by X-ray absorption spectroscopy: Fe(II) spin crossover complex

Photo-induced phase transitions in spin-crossover complex [Fe(II)(2-pic)₃]Cl₂EtOH were investigated by X-ray absorption spectroscopy (XAS) technique. In situ XAS results showed that the metal-ligand (Fe–N) bond distance expands by ca. 0.20 Å upon photo-induced (S=0→S=2) spin conversion. We find that nearly octahedral (O_h) symmetry of FeN₆ coordination remains in the photo-induced high spin state. The next-nearest neighbor Fe–C radial distribution shows a subtle change due to local distortions of ligand molecules. Inhomogeneous distribution of strains may disturb cooperative photo-induced phase transitions removing centers of inversion symmetry.     

Specific features of spin-variable properties of [Fe(acen)pic<Subscript>2</Subscript>]BPh<Subscript>4</Subscript> · <Emphasis Type="Italic">n</Emphasis>H<Subscript>2</Subscript>O

The [Fe(acen)pic₂]BPh₄ · nH₂O compound has been synthesized and studied in the temperature interval of 5–300 K by the methods of EPR and magnetic susceptibility. The existence of ferromagnetic interactions between Fe(III) complexes in this compound has been revealed, in contrast to unhydrated [Fe(acen)pic₂]BPh₄. The reduction in the integrated intensity of the magnetic resonance signal as the temperature decreases below 80 K has been explained by the transition of high-spin ions to the low-spin state. It has been shown that the phase transition temperature in the presence of intermolecular (ferromagnetic) interactions is lower than that in the case of noninteracting centers.     

Effect of N-substitution in multinuclear complexes allows interplay between magnetic states and multistability investigated by Mössbauer spectroscopy

A series of pentadentate ligands N–X–⁵LH₂ (X?=?H, Methyl, Benzyl)?=?N–X–saldptn (4-X-N,N′-bis(1-hydroxy-2-benzylidene)-1,7-diamino-4-azaheptane) has been prepared by a Schiff base condensation between 1,7-diamino-4-X-azaheptane and salicylaldehyde. Complexation with Fe(III) yields a series of high-spin (S?=?5/2) complexes of [Fe^III(N–X–⁵L)Cl]. Such precursors were combined with [Mo(CN)₈]^4? and a series of blue nonanuclear cluster compounds [Mo^IV{(CN)Fe^III(N–X–⁵L)}₈]Cl₄ resulted. Such star-shaped nonanuclear compounds are high-spin systems at room temperature. On cooling to 10 K some of the iron(III) centers switched to the low-spin state as proven by Mössbauer spectra, i.e. multiple electronic transitions. Parts of the compounds perform a high-spin to high-spin transition. Under light irradiation the populations are altered slightly.     

Magnetic properties of nanoparticles of Prussian blue-based molecular magnets M 3[Cr(CN)6]2⋅zH2O (M=Fe, Co, and Ni)

The nanoparticles of Prussian blue-based molecular magnets, M ₃[Cr(CN)₆]₂?zH₂O (where M=Fe, Co, and Ni), prepared by a slow addition (drop by drop) of chemicals using the co-precipitation method, are investigated by means of X-ray diffraction, infra red spectroscopy and dc magnetization measurement techniques. The formation of nanoparticles has been confirmed by scanning electron microscopy, whereas the characteristic peak, observed in the range of 1900–2300 cm^?1 in the infrared spectra, corresponds to the CN stretching frequency of $\mbox{Cr}^{\mathrm{+III}}$ –CN– $M^{\mathrm{+II}}$ , and confirms the formation of Prussian blue compounds. The results, derived from the Rietveld refinement of X-ray diffraction patterns, reveal that all samples are nanocrystalline in nature with a face-centered cubic crystal structure of space group Fm3m. The particle size and the lattice constants decrease with an increasing atomic number of the transition metals (M=Fe, Co and Ni). The magnetization data show a magnetically ordered state of all nanoparticle samples with a low coercivity (except for the Fe₃[Cr(CN)₆]₂?zH₂O) as well as the remanent magnetization. In addition, by varying M with Fe, Co and Ni, the magnetic ordering temperature increases from ～12 to ～28 K, whereas the maximum magnetization and the coercive field decrease from ～14 to ～4.5 μ_B/f.u. and ～554 to ～22 Oe, respectively. The observed magnetization behavior has been discussed in terms of the structural changes due to the decreasing particle size as well as the varying nature of the metal ions.     

Characterization of T-55 sorbent using 57Fe Mössbauer spectroscopy with a high velocity resolution

A new sorbent T-55 with mixed ferrocyanide surface modification developed for Cs⁺ sorption was characterized using Mössbauer spectroscopy with a high velocity resolution in comparison with K₂Ni[Fe(CN)₆] and K₄[Fe(CN)₆ samples. The main sorption active component of T-55 sorbent was determined as K₂Ni[Fe(CN)₆].     

Electrochemical properties of compacts of nano-and microdisperse diamond powders in aqueous electrolytes

The electrochemical behavior of compacts of micro-and nanodisperse diamond powders were studied by using model redox K₃[Fe(CN)₆]-K₄[Fe(CN)₆] and Ce(SO₄)₂-Ce₂(SO₄)₃ systems in aqueous electrolytes. The current-voltage curves for compacts of microdisperse diamonds and the kinetics of reactions on these compacts in a solution of the [Fe(CN)₆]^3-/4- system are similar to those obtained by using a metal electrode. For nanodisperse diamonds, the same reactions are essentially irreversible.     

High‐pressure studies of the micro‐Raman spectra of iron cyanide complexes: Prussian blue (Fe4[Fe(CN)6]3), potassium ferricyanide (K3[Fe(CN)6]), and sodium nitroprusside (Na2[Fe(CN)5(NO)]·2H2O)

The pressure dependences of the peaks observed in the micro‐Raman spectra of Prussian blue (Fe₄[Fe(CN)₆]₃), potassium ferricyanide (K₃[Fe(CN)₆]), and sodium nitroprusside (Na₂[Fe(CN)₅(NO)]·2H₂O) have been measured up to 5.0 GPa. The vibrational modes of Prussian blue appearing at 201 and 365 cm⁻¹ show negative dν/dP values and Grüneisen parameters and are assigned to the transverse bending modes of the Fe C N Fe linkage which can contribute to a negative thermal expansion behavior. A phase transition occurring between 2.0 and 2.8 GPa in potassium ferricyanide is shown by changes in the spectral region 150–700 cm⁻¹. In the spectra of the nitroprusside ion, there are strong interactions between the FeN stretching mode and the FeNO bending and the axial CN stretching modes. The pressure dependence of the NO stretching vibration is positive, 5.6 cm⁻¹ GPa⁻¹, in contrast to the negative behavior in the iron(II)‐meso‐tetraphenyl porphyrinate complex. Copyright © 2011 John Wiley & Sons, Ltd.     

Magnetic properties of substitutional solid solutions of nickel and iron hexacyanoferrate–hexacyanochromate

In order to evaluate the influence of substitution at the central metal ion position in transition hexacyanometallates in some detail, the magnetic studies were carried out on a series of solid solutions of metal hexacyanometallates of the composition M[Fe^II(CN)₆]_{1? x} [Cr^III(CN)₆] _x , where M?=?Ni^II and Fe^III. The temperature and field dependences of magnetization were studied using a superconducting quantum interference device magnetometer. The field dependence of the samples at 5?K shows a hysteresis behaviour. For M?=?Ni^II, the transition temperature increases with increase in the substitution of low-spin Fe(III) by Cr(III) in the hexacyanometallate unit. The saturation magnetization was found to decrease with increase in the iron concentration. The observed variation in the magnetic properties, such as the value of T _C and the nature of magnetic ordering, is attributed to the variation in the composition of the transition-metal ion in the coordination sphere of carbon. On the other hand, for M?=?Fe^III, the transition temperature and saturation magnetization remained almost unchanged, indicating that substitution at the carbon coordination site did not produce any change in the magnetic interaction among the transition-metal ions through the cyanide ions.     

Electrochemical Studies on Substituted Iron-Hexacyanoiron(III) Bi-Layered Thin Films at Glassy Carbon Electrode/Electrolyte Interface

Thin films (30–50 nm) of bilayered HCM (hexacyanometalates) were prepared using direct electrodeposition or electrochemically driven insertion-substitution of PB (Prussian Blue) or InHCF (Indium Hexacyanoferrate) or K..In _x [Fe{CN}₆] _y , as starting material. The redox behavior of the immobilized counter/central ions at GCE/electrolyte interface have been investigated in aqueous KCl (pH 1) electrolytes using dynamic voltammetric techniques. Studies show that when the counter Fe or In ion is replaced by expandable partially filled d orbits elements such as Ru³⁺ a redox wave of the inserted counter ion is observed. Furthermore, the substitution of Fe as a counter ion with other poly-valent cations was found to be reversible if PB was the starting material. Studies were extended to include the EC (electrochemical) behavior of related HCM compounds such as K...Al _x [Fe{CN}₆] _y K, Ni _x [Fe{CN}₆] _y Cu _x [Fe{CN}₆] _y , K..Zn _x [Fe{CN}₆] _y and, K..Ru[Fe{CN}₆]₃. Unlike studied 3d cations, GCE modified with thin films of copper-hexacyanoiron (III) KCu _x [Fe{CN}₆] _y or CHF showed two redox waves with E^o _f 0.6, 0.9 V vs Ag/AgCl. Studies showed that even immobilized Cu ions were capable of catalyzing the oxidation of hydrazine and sulfite. The mechanism for electro-oxidation is also included.     

High-spin metal-cyanide clusters: species incorporating [Mn(salen)] complexes as a source of anisotropy

The use of N,N′-ethylenebis(salycylideneiminato) (salen) complexes of Mn^III in assembling high-spin metal-cyanide coordination clusters with significant magnetic anisotropy is demonstrated. The reaction of [Mn(salen)(H₂O)₂]⁺with [Cr(CN)₆]³⁻ in aqueous solution generates {Cr[CNMn(salen)(H₂O)]₆}[Cr(CN)₆]·6H₂O (1), a previously reported compound featuring a heptanuclear cluster with a distorted octahedral geometry. A fit to the variable-temperature magnetic susceptibility data for 1 revealed the presence of weak antiferromagnetic coupling of within the cluster, giving rise to an S=21/2 ground state. In addition, variable-field magnetization data collected at low temperatures revealed the presence of magnetic anisotropy in the ground state, with the best fit yielding zero-field splitting parameters of D=+0.19 cm⁻¹ and A reaction intended to produce a direct analogue of 1 by employing [Fe(CN)₆]³⁻ in place of [Cr(CN)₆]³⁻ instead gave an unusually complex compound of formula {Fe(CN)₄[CNMn(salen)(MeOH)]₂}{[Mn(salen)(H₂O)]₂}[Mn(salen)(H₂O)(MeOH)]₂[Fe(CN)₆]·4H₂O (2). The crystal structure and magnetic properties of this compound are reported.     

Spin dynamics in (( BEDO-TTF )6[ M (CN)6](H3O,CH3CN)2 ( M = Fe, Cr) crystals

Layered single crystals of the (BEDO-TTF)₆[M(CN)₆](H₃O,CH₃CN)₂ (M = Fe, Cr) compounds with alternating conducting layers of BEDO-TTF and [M(CN)₆](H₃O,CH₃CN)₂ are studied. The contributions to the magnetic susceptibility from charge carriers in BEDO-TTF layers and from the subsystem of localized magnetic moments of iron (or chromium) transition metal complexes are separated for both compounds under investigation. It is revealed that the crystals with [Fe(CN))₆]³⁻ anions at a temperature of ∼80 K and the crystals with [Cr(CN))₆]³⁻ anions at ∼30 K undergo magnetic transitions which are accompanied by drastic changes in the parameters of the EPR lines associated with the BEDO-TTF layers and the subsystem of localized spins of transition metal complexes. It is established that the presence of the BEDO-TTF layers in the structure affects the magnetic properties of iron and chromium hexacyanide complexes. Original Russian Text ? R.B. Morgunov, E.V. Kurganova, T.G. Prokhorova, E.B. Yagubskiĭ, S.V. Simonov, R.P. Shibaeva, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 4, pp. 657–663.     

Dynamics of the charge transferred states relevant to magnetic phase transition in rubidium manganese hexacyanoferrate

Photoinduced charge transfer dynamics in the photomagnetic material RbMn[Fe(CN)₆], which exhibits a magnetic phase transition with a large hysteresis loop (230-300 K), has been investigated by observing the CN⁻ stretching modes, which are sensitive to the valences of the adjacent transition metal ions. Mid-infrared transient absorption measurements were performed between 2013 and 2179 cm⁻¹ to observe the transient and persistent products. The sample in the high-temperature phase was excited by 400 nm laser pulses at the ligand to metal charge transfer band near the high-temperature end of the hysteresis loop. Bleach of the Fe³⁺-CN⁻-Mn²⁺ band representing a decrease of the high-temperature phase and increases of the Fe²⁺-CN⁻-Mn³⁺ and Fe²⁺-CN⁻-Mn²⁺ bands were observed in picosecond time region, indicating a transient production of charge transferred states.     

On bistability in molecular systems: Photomagnetic studies of inorganic complexes

Photo‐induced effects have been detected by magnetic measurements, Mössbauer spectroscopy and reflectivity. The LIESST effect has been achieved in the spin‐crossover system [Fe_xCo_1-x(btr)₂(NCS)₂]·H₂O. We investigated the purely photo‐induced magnetism of a Prussian Blue analogue Rb_0.52Co[Fe(CN)₆]_0.84, 2.31 H₂O, involving an optical electron transfer from Fe^II to Co^III. Inherent aspects of photomagnetic experiments are described: bulk and surface effects, magnetic and electronic metastabilities of the photo‐excited state.     

Ab initio study of the electronic structure of the molecule-based ferromagnet Co[N(CN)2]2

We have investigated the electronic and magnetic properties of the molecule-based magnet Co[N(CN)₂]₂ using the full potential linearized augmented plane wave (FP-LAPW) method. The relative stability of the ground state, density of states and charge distributions were examined. Total energy calculations reveals that the ferromagnetic phase is a stable ground state for Co[N(CN)₂]₂ in agreement with the previous experimental findings. It is noteworthy that we predict the Co[N(CN)₂]₂ is a ferromagnetic semiconductor with a small band gap of 0.027 eV, and the semiconductor property can be connected to the strong crystal field splitting of Co²⁺ 3d states for Co[N(CN)₂]₂. Such a molecule-based ferromagnetic semiconductor would offer a potential for semiconductor applications, therefore, an experimental confirmation of our theoretical predictions is encouraged.     

Vibrational spectra of crystalline potassium ferri- and ferrocyanide

The IR spectra of the crystalline salts K₃[Fe(CN)₆] (I) and K₄[Fe(CN)₆] (II) have been studied in the 1600–5000 cm^–1 range for the purpose of investigating the spectral difference in the valence states of the central atoms of the complex anions. The appearance of the doublet structure of thev ₆ band in the spectrum of salt II is due to distortion of its anion to D_4h symmetry. The vibrational spectra of both salts are interpreted.In conclusion, I should like to express ray sincere appreciation to Academician A. N. Terenin for his guidance and to L. A. Grivov and D. S, Bystrov for their valuable comments.     

Application of Mössbauer spectroscopy to the thermal decomposition of strontium and barium bis(CITRATO)Ferrates(III)

The thermal decomposition of M₃[Fe(cit)₂]₂ 4H₂O (M = Sr, Ba) has been studied isothermally and non-isothermally employing simultaneous (TG–DTG–DSC), XRD, IR, SEM and Mössbauer spectroscopic techniques. The anhydrous complexes decompose at 140–220°C to yield α-Fe₂O₃ and MC₅H₄O₅ (metal acetone-dicarboxylate) intermediates. The latter decomposes to respective carbonate at higher temperature. α-Fe₂O₃ and MCO₃ undergo a solid state reaction in the temperature range 400–430°C to yield Sr₂Fe₂O₅ and BaFe₂O₄ respectively. Finally above 600°C, a solid state reaction between these products and respective metal oxides leads to the formation of M₃Fe₂O_7???X showing simultaneous presence of Fe(III) and Fe(IV) species. These perovskite ferrites can exhibit electrical conductivity and dielectric properties. SEM analysis shows these ferrites to be nanosized with average particle diameter of 50–55 nm.