High-spin Phase Research Articles

Investigation of nucleation and growth phenomena during the thermal and light induced spin transition in the [Fe(1-bpp)2][BF4]2 complex

Abstract Optical microscopy measurements have been realized on single crystals of the [Fe(1-bpp)2][BF4]2 complex (1-bpp=2,6-di(pyrazol-1-yl)pyridine). The thermal spin transition around 253 K occurs by a heterogeneous nucleation and growth mechanism and involves a clear phase separation and a small hysteresis. This very abrupt and complete thermal transition is preceded by a premonitory spin conversion, which implies only a small fraction (ca. 2–3 %) of the molecules. This peculiar behavior may be the sign of heterophase fluctuations. The light-induced spin transition from the stable low spin (LS) to the metastable high spin (HS) phase was achieved at 80 K by focusing laser light into a small volume fraction of the crystal. Under continuous irradiation this photo-converted HS “nucleus” then grows and the whole crystal converts to the HS phase providing evidence for a light-induced instability in the system due to long-range elastic interactions. The relaxation of the light-induced metastable HS phase at 83 K follows a sigmoidal decay – typical of cooperative spin crossover systems. Nevertheless the spatial development of this relaxation process appears very homogeneous and no phase separation could be detected within the resolution of the optical microscope.

Analysis of Architecture Effect on Hysteretic Behavior of 3-D Spin Crossover Nanostructures

We have analyzed the hysteretic properties of thermal and pressure induced low spin (LS)↔high spin (HS) phase transition of a 3-D spin crossover (SCO) nanostructure when the cluster architecture is progressively changed from a cubic to a thin film. For solving the 3-D Ising-like Hamiltonian, the Monte Carlo metropolis technique is used, with the edge molecules trapped in HS state. The role of short- and long-range interactions on the SCO cooperativity has been investigated as well. The obtained results show that the cooperativity of a 3-D SCO system, characterized by the same number of molecules and the same interactions strength are strongly affected when going from a cubic 3-D to thin-film architecture. These results are discussed in terms of surface effects.

Pressure-induced spin collapse of octahedrally coordinatedFe3+inCa3Fe2[SiO4]3from experiment and theory

The high-pressure behavior of andradite garnet, ${\mathrm{Ca}}_{3}{\mathrm{Fe}}_{2}{\mathrm{[SiO}}_{4}$]${}_{3}$, was studied at pressures up to 80 GPa using single-crystal synchrotron x-ray diffraction, Raman spectroscopy, and quantum mechanical calculations based on density functional theory. An isosymmetric phase transition was observed in the pressure range between 60 and 70 GPa, which is associated with a gradual high-spin to low-spin electronic transition in ${\mathrm{Fe}}^{3+}$. Experimental structural data before and after the phase transition are in excellent agreement with the theoretically predicted structural compression of the high-spin and low-spin phases, respectively. While the overall unit-cell volume is reduced by about $2.5%$ across the phase transition, a collapse of about $10%$ of the ${\mathrm{FeO}}_{6}$ octahedral volume is observed, attributed to the reduced Fe--O bond lengths associated with low-spin ${\mathrm{Fe}}^{3+}$. In combination with earlier data the present study shows that a spin collapse of ${\mathrm{Fe}}^{3+}$ in ${\mathrm{FeO}}_{6}$ octahedra will be triggered if the Fe--O bond length reaches a critical value of $d$(Fe--O) $<$ $\ensuremath{\approx}$ 1.9 \AA{}. Earlier reported results for the compressibility of the cation coordination polyhedra are substantially revised, with ${B}_{{\mathrm{FeO}}_{6}}=195(2)$ GPa, ${B}_{{\mathrm{CaO}}_{8}}=104(2)$ GPa, and ${B}_{{\mathrm{SiO}}_{4}}=348(11)$ GPa for the high-spin phase. The mode Gr\"uneisen parameters range between 0.61 and 1.34. The computed spin-pairing energy is $\ensuremath{\approx}4.2$ eV at 0 GPa.

Thermal equation of state and spin transition of magnesiosiderite at high pressure and temperature

In situ synchrotron X-ray diffraction experiments on natural magnesiosiderite [(Mg0.35Fe0.65)CO3] were conducted using resistive and laser-heated diamond-anvil cells (DACs) up to 78 GPa and 1200 K. Based on thermal elastic modeling of the measured pressure-volume curves at given temperatures, we have derived thermal equation of state (EoS) parameters and the spin-crossover diagram of magnesiosiderite across the spin transition. These results show the spin crossover broadened and shifted toward higher pressures at elevated temperatures. Low-spin magnesiosiderite is 6% denser and 8% more incompressible than the high-spin phase at 1200 K and high pressures. Within the spin crossover from 53 to 63 GPa at 1200 K, magnesiosiderite exhibits anomalous thermal elastic behaviors, including a dramatic increase in the thermal expansion coefficient by a factor of 20 and a drop in the isothermal bulk modulus and the bulk sound velocity by approximately 75 and 50%, respectively. Compared with the end-member magnesite [MgCO3] at relevant pressure-temperature conditions of the subducted slabs, the high-spin magnesiosiderite with 65 mol% FeCO3 is approximately 21-23% denser and its unit-cell volume is 2-4% larger, whereas the low-spin state is 28-29% denser and 2% smaller than the end-member magnesite. Since ferromagnesite with 20 mol% of iron has been proposed to be a potential deep-carbon carrier, our results here indicate that the dense low-spin ferromagnesite can become more stable than high-spin ferromagnesite at pressures above approximately 50 GPa, providing a mechanism for (MgFe)-bearing carbonate to be a major carbon host in the deeper lower mantle.

Unprecedented bistability domain and interplay between spin crossover and polymorphism in a mononuclear iron(ii) complex

A mononuclear complex, [FeL2](BF4)2·xH2O (1LS∙xH2O), where L is 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine, can be converted into several phases showing different spin crossover regimes. In the first heating 1LS∙xH2O loses water molecules and converts into a mixture of two high spin phases, 1A/HS and 1C/HS. Further cycling produces the low spin phase 1A/LS. The transition 1A/LS↔1A/HS is accompanied by a 130 K wide hysteresis loop (Tc↑ = 490 K, Tc↓ = 360 K). Annealing the complex 1LS∙xH2O at 500 K yields a high spin phase 1C/HS. The phase 1C/HS undergoes spin conversion to the corresponding low spin phase 1C/LS on cooling, T1/2 ≈ 320 K. Dehydration of 1LS∙xH2O at 370 or 400 K yields a certain low spin phase, 1X/LS, which irreversibly transforms into a high spin phase 1B/HS, which, in turn, reversibly transforms to the low spin phase 1B/LS on cooling, T1/2 ≈ 320 K.

Unprecedented Multi‐Stable Spin Crossover Molecular Material with Two Thermal Memory Channels

Two in one: A new iron(II) complex with short alkyl substituents exhibits an unprecedented bimodal behavior governed by the coexistence of three phases: two structurally different low-spin phases and one high-spin phase. The compound features two distinct well-separated strong cooperative spin-crossover transitions by varying the scan rate (see graphic).

Study of the fast photoswitching of spin crossover nanoparticles outside and inside their thermal hysteresis loop

We have studied the low spin to high spin phase transition induced by nanosecond laser pulses outside and within the thermal hysteresis loop of the [Fe(Htrz)2 trz](BF4)2-H2O spin crossover nanoparticles. We demonstrate that, whatever the temperature of the compound, the photo-switching is achieved in less than 12.5 ns. Outside the hysteresis loop, the photo-induced high spin state remains up to 100 μs and then relaxes. Within the thermal hysteresis loop, the photo-induced high spin state remains as long as the temperature of the sample is kept within the thermal loop. A Raman study indicates that the photo-switching can be completed using single laser pulse excitation.

Monte Carlo Metropolis study of cluster evolution in spin-crossover solids within the framework of a mechanoelastic model

In this paper we apply a standard Monte Carlo Metropolis procedure for the treatment of a mechanoelastic model, in order to study the evolution of clusters in open boundary hexagonal spin-crossover systems, composed of molecules in a triangular lattice and linked by springs. The temperature-driven transition between the paramagnetic high-spin phase and the diamagnetic low-spin phase is reproduced by taking into account the lattice elastic energy change in addition to the energy variation of spin-active molecules. This method allows us to determine where higher amounts of energies are stored and where the spins are more probable to flip and to form clusters. The spreading of clusters from the corners, similar to recently published experimental data, is well reproduced. We also analyze here the lattice deformation during the transition and we show that the deformation increases as the spring constant diminishes. A systematic comparison with results obtained in the framework of our previous Arrhenius approach of the mechanoelastic system is provided.

Lattice-Mediated Propagation of Photoinduced Phase Transition in Co–Fe Cyanide

Spin-crossover compounds show many photoinduced phase transition (PIPT) phenomena, reflecting the strong coupling between the electron, spin, and lattice degrees of freedoms. In these compounds, the photoinduced electronic excitation triggers the intersystem of crossing in Fe2þ or Co3þ with a high probability, and alters the spin state of the transition metals (light-induced excited spin state trapping: LIESST). The spin state transition further causes local lattice deformation as a result of the variation in the ionic radii of Fe2þ or Co3þ.14) The resultant local lattice distortion modifies the electronic structure of the surrounding sites, causing a nonlinear response of the system. Ogawa et al. reported the threshold and incubation behaviors in the photoinduced phase transition of [Fe(2-pic)3]Cl2 EtOH (2-pic = 2-aminomethyl-pyridine) at 2.2K. Liu et al. reported a dynamical phase transition in Nafion–[Fe(Htrz)3] (Htrz = 1,2,4-4H-triazole) and an optical hysteresis in [Fe(pyz)6](BF4)2 (ptz = 1-propyltetrazole) at 77K. Several researchers theoretically investigated the photoinduced dynamics beyond the mean-field approximation, even though the approach had been successful in several situations. In this Note, we reported the mesoscale lattice effect on PIPT of a Co–Fe cyanide, Na0:27Co[Fe(CN)6]0:713.8H2O, at 200K by a microscopic spectroscopy. The compound shows a spin-crossover-type PIPT from the low-spin (LS) phase with LS Co3þ and LS Fe2þ to the high-spin (HS) phase with HS Co2þ and LS Fe3þ, accompanying a significant lattice expansion of 3%. We carefully investigated the PIPT dynamics at each position around the photoexcitation spot, which is 75 m in diameter. We found that the PIPT at each position takes place simultaneously within 0.2 s, even though the density (nHS) of the HS sites is widely distributed in the range of 0–0.6. The distribution of the linear expansion coefficient ( L=L) within the HS region suggests that the propagation of the PIPT region is mediated by a coherent lattice expansion. A thin film of Na0:27Co[Fe(CN)6]0:713.8H2O, 1310 nm in thickness, was electrochemically synthesized on an indium–tin oxide (ITO) transparent electrode. The chemical composition was determined by inductively coupled plasma (ICP) and CHN organic elementary analyses. The Xray diffraction pattern revealed that the compound is facecentered cubic with a lattice constant of a 1⁄4 10:34 A. The surface and cross-sectional scanning electron microscopy (SEM) images revealed that the film consists of rodlike crystals, 600 nm in length and 130 nm in area. The film shows a thermally induced LS–HS phase transition at 230K (1⁄4 Tc) in the warming run, and the film changes from violet (LS phase) to red (HS phase). The photoinduced dynamics was recorded with a laboratory-built microscopy system equipped with a charge-coupled device (CCD) camera. We used the second harmonics (532 nm) of the yttrium aluminum garnet (YAG) laser as the excitation light source. The spot size of the excitation light was 75 m, and the excitation density was 6.6 photon s 1 site . A notch filter was inserted to eliminate the scattering from the excitation light. The magnitudes of nHS at each time and position were determined by the chromaticity in the microscopy moving image. The spatial resolution of the system was 5 m. To increase the signal-to-noise ratio, the chromaticity was averaged within the 10 10 pixels around each position. Figure 1 shows examples of the microscopy images of the Na0:27Co[Fe(CN)6]0:713.8H2O film under photoirradiation at 200K at various times. The bright region corresponds to the HS region. No macroscopic PIPT is observed at (b) 41.7 s. The PIPT starts around the central region of the excitation spot at (c) 43.3 s, and then expands outside at (d) 43.5 s. A similar incubation behavior of the PIPT was observed in [Fe(2-pic)3]Cl2 EtOH. One may notice that the PIPT region at (d) 43.5 s extends beyond the excitation spot [broken-line circle in Fig. 1(a)]. The inset of Fig. 2(b) shows a prototypical example of the time dependence of nHS. (a) 0 .0 s

On the Role of Intermolecular Interactions on Structural and Spin‐Crossover Properties of 2D Coordination Networks [Fe(bbtr)3]A2 (bbtr=1,4‐bis(1,2,3‐triazol‐1‐yl)butane; A=ClO4−, BF4−)

A series of complexes [M(bbtr)(3)]A(2) (M=Fe(II), Zn(II); bbtr=1,4-bis(1,2,3-triazol-1-yl)butane; A=ClO(4)(-), BF(4)(-)) and [Fe(x)Zn(1-x)(bbtr)(3)](ClO(4))(2) (0<x<1) dilute systems was synthesized and characterized. Earlier studies on [Fe(bbtr)(3)](ClO(4))(2) (1⋅ClO(4)), which crystallizes in space group P3, revealed an abrupt spin transition with T(1/2)↓ ≈101 K and T(1/2)↑ ≈109 K. Variable-temperature measurements of the lattice parameters and change in Bragg peaks profiles of 1⋅ClO(4) showed a structural phase transition at 125 K leading to space group P1. Single-crystal X-ray diffraction studies allowed structures of the quenched high-spin phase and then a low-symmetry low-spin phase of 1⋅ClO(4) to be determined. We established that tetrafluoroborate analogue 1⋅BF(4) remains in the high-spin form in the temperature range 5-300 K. Contrary to 1⋅ClO(4), structural investigations did not reveal the presence of a structural phase transition in 1⋅BF(4). Analogous to 1⋅ClO(4), [Zn(bbtr)(3)](ClO(4))(2) (2⋅ClO(4)) undergoes a structural transition at 151 K, whereas at low-temperature [Zn(bbtr)(3)](BF(4))(2) (2⋅BF(4)) remains in space group P3. The structural phase transitions in both perchlorates are accompanied by similar reorganization of the intermolecular contacts, which leads to a shift of 2D layers. In effect, compression of the unit cell takes place, favouring the appearance of the spin transition in 1⋅ClO(4) in relation to 1⋅BF(4). The metal ion determines the temperature of the structural phase transition in 1⋅ClO(4) and 2⋅ClO(4). This property was exploited to separate the non-magnetic structural transformation and spin transition in 1⋅ClO(4). The spin and structural transitions in the [Fe(x)Zn(1-x)(bbtr)(3)](ClO(4))(2) systems take place at different temperatures, and the temperature difference between them increases with increasing zinc(II) content. This means that structural phase transition is necessary for appearance of the spin transition, but this transformation does not trigger directly the spin transition in 1⋅ClO(4).

Crystallization Process of Photoexcited High-Spin Sites in Co–Fe Cyanide Film

The dynamics of the photoinduced phase transition (PIPT) was systematically investigated in Co–Fe cyanide film, Na 0.16 Co[Fe(CN) 6 ] 0.71 5.8H 2 O, at 160 and 190 K above the melting temperature ( T m = 120 K) of the photoinduced phase. Below T m , the film shows a permanent photoinduced phase transition from a low-spin (LS) phase to a static high-spin (HS) phase. We found that the relaxation curve from the photo-excited state changes from the exponential type to Avrami's type as the excitation photon density ( n ) increases. We interpreted this behavior in terms of crystallization process of the photo-created HS sites.

Nanoscale Self‐Hosting of Molecular Spin‐States in the Intermediate Phase of a Spin‐Crossover Material

A new spin-crossover (SC) complex [Fe(II)H(2)L(2-Me)][AsF(6)](2) has been synthesized, in which H(2)L(2-Me) denotes the chirogenic hexadentate N(6) Schiff-base ligand bis{[(2-methylimidazol-4-yl)methylidene]-3-aminopropyl}ethylenediamine. This complex has revealed a rich variety of phases during its two-step thermal crossover, as well as photoinduced spin-state switching. A high-symmetry high-spin (HS, S=2) phase, a low-symmetry low-spin (LS, S=0) phase, an intermediate phase characterized by an unprecedented lozenge pattern of 12 predominantly HS molecular crystallographic sites confining 18 predominantly LS molecular crystallographic sites, and a photoinduced low-symmetry HS phase have been accurately evidenced by temperature-dependent magnetic susceptibility, Mössbauer spectroscopy, and crystallographic studies. This variety of phases illustrates the multi-stability of this system, which results from coupling between the electronic states and structural instabilities.

Photoinduced phase separation in spin-crossover materials: Numerical simulation of a dynamic photocrystallographic experiment

The dynamics of photoinduced phase transition in spin-crossover materials is studied using numerical simulations of a microscopic two-variable anharmonic model, in close connection with photocrystallographic experiments on the prototype system $[\text{Fe}{(\text{btr})}_{2}{(\text{NCS})}_{2}].{\text{H}}_{2}\text{O}$. A comparative analysis of the simulated diffraction pattern with excitation duration and intensity as variables is performed. Nonlinear dynamics, threshold effect in excitation intensity and light-induced phase separation are modeled and attributed to a strong electron-lattice coupling, mediated by long-range elastic interactions. Photoinduced nucleation and domain growth of the metastable high-spin phase is evidenced and quantitatively explained in the Avrami formalism.

Pressure-Driven Metal-Insulator Transition in Hematite from Dynamical Mean-Field Theory

The local density approximation combined with dynamical mean-field theory is applied to study the paramagnetic and magnetically ordered phases of hematite Fe2O3 as a function of volume. As the volume is decreased, a simultaneous first-order insulator-metal and high-spin to low-spin transition occurs close to the experimental value of the critical volume. The high-spin insulating phase is destroyed by a progressive reduction of the spectral gap with increasing pressure, upon closing of which the high-spin phase becomes unstable. We conclude that the transition in Fe2O3 at approximately 50 GPa can be described as an electronically driven volume collapse.

Two new 4′,4′′-disubstituted dipyrazolylpyridine derivatives, and the structures and spin states of their iron(II) complexes

Reaction of 2 equiv of the sodium salt of ethyl pyrazole-4-carboxylate, with 1 equiv of 2,6-dibromopyridine, in diglyme at 130 °C for 5 days yields 2,6-di[4-(ethylcarboxy)pyrazol-1-yl]pyridine (L 1), with 2-bromo-6-[4-(ethylcarboxy)pyrazol-1-yl]pyridine (L 2) as a significant byproduct. Reduction of L 1 with excess NaBH 4 in thf affords 2,6-di[4-(hydroxymethyl)pyrazol-1-yl]pyridine (L 3) in low yield. The crystalline complex [Fe(L 1) 2][BF 4] 2 · 2CF 3CH 2OH is low-spin at 150 K, while bulk samples with this formula are approximately 10% high-spin and 90% low-spin at room temperature. This ratio does not vary significantly on cooling from its magnetic susceptibility, suggesting that the material might be contaminated by a second, minor high-spin phase. Single crystals of [Fe(L 3) 2][BF 4] 2·1.4CH 3CN have a mixed spin-state population, with the low-spin state predominating at 150 K. The [Fe(L 3) 2(BF 4)] + moieties in the lattice associate into 1-D chains through intermolecular O–H⋯O and O–H⋯F hydrogen bonding. Bulk samples of [Fe(L 3) 2][BF 4] 2 · H 2O are fully low-spin below 200 K, but the magnetic data imply the onset of a gradual thermal spin-transition centred above room temperature. DSC and TGA measurements imply that this transition is centred at 322 K, and involves loss of lattice water. Both complexes undergo spin-crossover in (CD 3) 2CO solution, with transition midpoints near 250 K.

The pressure-induced spin transition in the Fe(phen)2(NCS)2 model compound

The influence of pressure on the high spin-low spin phase transition (HL transition) in a model Fe(phen)2(NCS)2 (polymorph II) compound at room temperature was studied by optical spectroscopy. An increase in pressure from atmospheric to 1.814 hPa caused the complete conversion of the low-spin to high-spin phase with the transition pressure p1/2 ↑ = 0.567 hPa at the equilibrium concentration of the high- and low-spin phases. Pressure drop caused the reverse HL transition with p1/2 ↓ = 0.543 hPa. The HL reversible transition took place with the transition pressure p1/2 = 0.555 hPa and hysteresis with width Δp1/2 = 0.024 hPa. The p1/2 value for the pressure-induced HL transition corresponded to the T1/2 value under pressure for the temperature-induced HL transition. The pressure dependences of the fraction of the high-spin phase determined from four independent measurements of the influence of pressure on the temperature-induced HL transition and the pressure-induced HL transition obtained in this work were compared. The experimental data were used to calculate the interaction and elastic energy parameters for temperature- and pressure-induced HL transitions. The qualitative coincidence of the interaction parameters for pressure- and temperature-induced HL transitions and the equality of the interaction parameter to elastic energy under pressure led us to conclude that the influence of pressure in two experiments well corresponds to the interaction energy and does not correspond the Gibbs potential.

Prediction of half-metallic conductivity in Prussian Blue derivatives

Half-metallicity and spin-crossover behavior are predicted for two well-defined Prussian Blue (PB) analogues using periodic density functional calculations with GGA + U and hybrid exhange-correlation potentials. Fully oxidized PB possesses a narrow spin-polarized band through which it should be possible to control the spin conduction by oxidative/reductive doping. Fully reduced PB is also a half-metal in its high-spin phase and exhibits an Fe2+ spin-crossover. A simple model is used to rationalize the electronic properties of these systems which may have potential applications in spintronics.

Spin Crossover in a Series of Iron(II) Complexes of 2-(2-Alkyl-2H-tetrazol-5-yl)-1,10-phenanthroline: Effects of Alkyl Side Chain, Solvent, and Anion

2-(2H-Tetrazol-5-yl)-1,10-phenanthroline (HL0), its alkyl-substituted derivatives (Ln, where n = 1-8, 10, 12, 14, and 16, denoting the carbon atom number of the alkyl chain) at the 2H position of the tetrazole ring, and their iron(II) complexes (a for [Fe(L0)2], na for [Fe(Ln)2](ClO4)2, and nb for [Fe(Ln)2](BF4)2) were synthesized and characterized. The crystal structures of a, a.CH3OH, 1a.CH3OH, 1b.CH3OH.CH3CN, 2a.H2O, 2b.H2O, 4b.CH3OH, 5a.H2O, 5b.H2O, 6a, 6b, 7a, 7b, and 16a are described, along with thermal analyses. a undergoes an abrupt spin crossover (SCO) at 255 K with a hysteresis loop of 6 K. a.CH3OH, 2a.H2O, and 2b.H2O exhibit irreversible SCO behaviors due to the loss of solvent molecules upon heating. 3a, 3b, 4a, and 5a.H2O show simple spin transitions above 350 K. The desolvated samples of 4b.CH3OH and 5b.H2O undergo two-step spin transitions. 16a exhibits a two-step SCO behavior between 100 and 300 K, corresponding to sequential phase transitions from the low-spin (LS) phase to the intermediate phase and then to the high-spin phase, respectively, proved by crystal structure analysis and 57Fe Mössbauer spectroscopy. 1a.CH3OH, 10a, 10b, 12a, 12b, 14a, 14b, and 16b show gradual and incomplete SCO behaviors after cooling down from 400 K. 1b.CH3OH.CH3CN, 6a, 6b, 7a, 7b, 8a, and 8b remain in the LS state even at 400 K. This proves that the alkyl side chains, together with the solvent molecules and anions, play a crucial role in the complicated SCO behaviors in this system.

Character of the excited state of theCo3+ ionin LaCoO3

Generalized gradient approximation(GGA)+U calculationsfor LaCoO3 confirm the existence of a stationary state composed of a mixture of the low spin (LS) and high spin(HS) Co3+ ions in a 1:1 ratio, which is of insulating character. At low temperatures,this state is located about 25 meV above the homogeneous LS ground state ifCoLS–O–CoHS bondlength optimization is taken into account. The energy difference decreases with the lattice dilatation andthe LS+HS(1:1) phase with bond length optimization becomes stable at some intermediate temperature. Theseresults, together with previous electron spin resonance evidence for HS excitations in theLS ground state, lead to the conclusion that the diamagnetic–paramagnetic transition inLaCoO3 at 50–150 K is caused by a gradual population of HSCo3+ ionic states, provided that neighbouring sites are in the LS state. Consistency betweenexperimental data for the paramagnetic susceptibility and anomalous thermal expansion isachieved for an HS–LS energy difference of 16 meV that corresponds to 180 K. With increasingHS population, the antiferromagnetic (AF) interactions HS–LS–HS become effective. TheLS+HS(1:1) phase is saturated above 150 K and the susceptibility acquires a simpleCurie–Weiss behaviour in which AF interactions are characterized byθ = −160 K.

Synchrotron radiation study of photo-induced spin-crossover transitions: Microscopic origin of nonlinear phase transition

In situ X-ray absorption spectroscopy (XAS) using synchrotron radiation was used to probe the local structure of photo-induced phase transition of Fe (II) spin-crossover complex. We demonstrate that the local structure of spin-converted metastable phase can be probed by XAS using a novel pixel array detector using less than 1 mg of specimen. We find that the arrangement of nearest neighbors, i.e., the FeN 6 cluster does not change its symmetry upon the diamagnetic ( S = 0 ) ↔paramagnetic ( S = 2 ) transformation under laser light (532 nm) excitation below 50 K. The intermediate-range structure (next-nearest neighbor correlation) shows that ligand molecules in the photo-induced high spin phase is distorted. The results suggest that the essential difference between the photo-induced and thermally induced high spin phases is the absence of intermediate-range order in the former. We propose that quenched disorder (inhomogeneity) in the intermediate-range structure is the microscopic origin of nonlinear nature of photo-induced phase transition (crippled cooperativity). It is demonstrated that in situ XAS using synchrotron radiation and state-of-the-art X-ray detector can provide detailed information on the local structure of metastable states trapped at low temperature.