Spin-crossover Phase Transition Research Articles

Temperature-scan-rate dependent SCO hysteresis found in a tetramethylphosphonium tripodal ferrate(II) salt

A new compound (Me4P)[Fe(py3COH)(NCS)3]•MeOH was prepared and structurally characterized, where py3COH stands for tris(2-pyridyl)methanol. After annealed at 400 K, desolvated (Me4P)[Fe(py3COH)(NCS)3] displayed an abrupt spin-crossover (SCO) at T1/2↑ = 255 K and T1/2↓ = 234 K. Namely, 21 K-wide hysteresis was recorded at a temperature scan rate of 5 K min−1. The χmT vs T profile was temperature-scan rate dependent; the wider hysteresis with the slower scan. The secondary SCO with T1/2↑ = 275 K emerged, and a new concomitant hysteresis loop overlapped the primary one. The total hysteresis width became 39 K at a slow scan with 0.1 K min−1. A mechanism of interplaying spin crossover and structural phase transition is described. Combining a similar result on the corresponding Me4N+ analog, we supposed that the tripod py3COH ligand would have an advantage for hysteretic SCO and possibly for double SCO behavior.

Static and dynamic properties of two-sublattice spin-crossover systems

We investigate the static and dynamic properties of spin-crossover systems in the presence of an external magnetic field using two-sublattice Ising-like Hamiltonian. At low temperatures both sublattices are in the LS state, and in the HS state at high temperatures. The new feature of spin-crossover is depicted for our model when we take the non-equivalent sublattices JA≠JB, the two-step spin-crossover phase transition is obtained with the presence of the hysteresis loop. In order to study the properties of two-step spin-crossover, we use the lowest approximation of the cluster variation method for static properties. The dynamic properties of the system with bilinear interactions are studied by the path probability method. We present high-spin state fraction nHS vs temperature and magnetic field variations for constant values of the degeneracy ratio between high-spin and low-spin states. In the present model, we obtained the relaxation curves of the order parameters SA and SB using different temperature values. Finally, the flow diagram of the order parameters is depicted for different values of rate constant k1 and temperature, which displays the stable, metastable, and unstable properties. Static and dynamic self-consistent equations of the system were performed by using Newton-Raphson and Runge-Kutta methods.

Desolvation–Solvation-Induced Reversible On–Off Switching of Two Memory Channels in a Cobalt(II) Coordination Polymer: Overlay of Spin Crossover and Structural Phase Transition

Desolvation–Solvation-Induced Reversible On–Off Switching of Two Memory Channels in a Cobalt(II) Coordination Polymer: Overlay of Spin Crossover and Structural Phase Transition

Three Jahn-Teller States of Matter in Spin-Crossover System Mn(taa).

Three high-spin phases recently discovered in the spin-crossover system Mn(taa) are identified through analysis by a combination of first-principles calculations and MonteCarlo simulation as a low-temperature Jahn-Teller ordered (solid) phase, an intermediate-temperature dynamically correlated (liquid) phase, and an uncorrelated (gas) phase. In particular, the Jahn-Teller liquid phase arises from competition between mixing with low-spin impurities, which drive the disorder, and intermolecular strain interactions. The latter are a key factor in both the spin-crossover phase transition and the magnetoelectric coupling. Jahn-Teller liquids may exist in other spin-crossover materials and materials that have multiple equivalent Jahn-Teller axes.

Spin crossover and structural phase transition in homochiral and heterochiral Fe[(pybox)2]2+ complexes.

Spin crossover and structural phase transition in three pairs of homochiral and heterochiral [Fe(pybox)2]2+ diastereomers were investigated through magnetic and crystallographic studies. Herein, we show that the spin transition properties of these compounds are strongly related to the homochiral or heterochiral assembly of the ligands and structural phase transition. Our work demonstrates that chirality is a feasible factor to modulate and influence the molecular geometry and thus the spin-crossover properties of complexes.

First indirect experimental evidence and theoretical discussion of giant refrigeration capacity through the reversible pressure induced spin-crossover phase transition

We report on the giant barocaloric effect and refrigerant capacity in the [Fe(pzt)6](PF6)2 (pzt = 1-propyltetrazole) spin-crossover material. The refrigerant capacity in [Fe(pzt)6](PF6)2 is RC = 1380 J kg−1, 5 times higher than the big value reported in (NH4)2SO4, upon pressure variation ΔP = 1 kbar. This huge caloric effect is ascribed to the coupling interactions between the crystal lattice (phonons) and the order parameter (γHS) that describes the molar fraction of high spin molecules (Fe+2N6)-(t2g4eg2) in [Fe(pzt)6](PF6)2. Our theoretical entropy includes the lattice, electronic and configurational coupled-contributions and was obtained from a microscopic model. A new methodology to obtain the barocaloric effect potentials is presented using a proper thermodynamic Maxwell relation for spin-crossover systems. The experimental results, for the isothermal entropy change, were calculated from the pressure dependence of γHS data. Besides, the determination of molecular volume change between high and low spin states through caloric measurements was discussed.

Fe(II) spin-crossover phenomenon in the pentadecanuclear {Fe9[Re(CN)8]6} spherical cluster.

The self-assembly of iron(II) ions with rare octacyanidorhenate(V) metalloligands in a methanolic solution results in the formation of a nanometric pentadecanuclear {Fe(II) 9 [Re(V) (CN)8 ]6 (MeOH)24 }⋅10 MeOH (1) molecule with a six-capped body-centered cubic topology. The cluster demonstrates a thermally-induced spin-crossover phase transition at T1/2 =195 K which occurs selectively for a single Fe(II) ion embedded in the center of a cluster core.

Decoupled Spin Crossover and Structural Phase Transition in a Molecular Iron(II) Complex

Crystalline [Fe(bppSMe)2][BF4]2 (1; bppSMe = 4-(methylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine) undergoes an abrupt spin-crossover (SCO) event at 265±5 K. The crystals also undergo a separate phase transition near 205 K, involving a contraction of the unit-cell a axis to one-third of its original value (high-temperature phase 1; Pbcn, Z = 12; low-temperature phase 2; Pbcn, Z = 4). The SCO-active phase 1 contains two unique molecular environments, one of which appears to undergo SCO more gradually than the other. In contrast, powder samples of 1 retain phase 1 between 140-300 K, although their SCO behaviour is essentially identical to the single crystals. The compounds [Fe(bppBr)2][BF4]2 (2; bppBr = 4-bromo-2,6-di(pyrazol-1-yl)pyridine) and [Fe(bppI)2][BF4]2 (3; bppI = 4-iodo-2,6-di(pyrazol-1-yl)-pyridine) exhibit more gradual SCO near room temperature, and adopt phase 2 in both spin states. Comparison of 1-3 reveals that the more cooperative spin transition in 1, and its separate crystallographic phase transition, can both be attributed to an intermolecular steric interaction involving the methylsulfanyl substituents. All three compounds exhibit the light-induced excited-spin-state trapping (LIESST) effect with T(LIESST = 70-80 K), but show complicated LIESST relaxation kinetics involving both weakly cooperative (exponential) and strongly cooperative (sigmoidal) components.

Pressure driven spin crossover and isostructural phase transition in LaFeO3

We have studied the behavior of LaFeO3 under pressure (P) using density functional theory (DFT) and atomistic simulations. Ground state structural properties of LaFeO3 are correctly described by atomistic simulations. The effect of high pressure shows that there is an isotropic compression up to 100 GPa. However, DFT calculations show that within pressure range 0 < P < 32.4 GPa, LaFeO3 retains its ground state electronic structure. On the other hand, at P ∼32.4 GPa high to low spin magnetic phase transition is observed, which is accompanied by 6.9% volume collapse of LaFeO3 unit cell, while retaining the ground state orthorhombic crystal structure, i.e., isostructural phase transition. Furthermore, the band gap is closed leading insulator to metal transition. This differing behavior observed by the two techniques can be attributed to the omission of magnetic effects in static simulations. The simultaneous magnetic, electrical, and structural (volume collapse) phase transitions of LaFeO3 under compression as revealed by DFT calculations corroborate experimental findings. From these results, we can elaborate the mechanism of phase transition in LaFeO3: increasing crystal field induces a high spin to low spin transition, which in turn drives the electrical transitions and volume collapse.

Giant Resistance Change across the Phase Transition in Spin-Crossover Molecules

The electronic origin of a large resistance change in nanoscale junctions incorporating spin-crossover molecules is demonstrated theoretically by using a combination of density functional theory and the nonequilibrium Green's function method for quantum transport. At the spin-crossover phase transition, there is a drastic change in the electronic gap between the frontier molecular orbitals. As a consequence, when the molecule is incorporated in a two-terminal device, the current increases by up to 4 orders of magnitude in response to the spin change. This is equivalent to a magnetoresistance effect in excess of 3000%. Since the typical phase transition critical temperature for spin-crossover compounds can be extended to well above room temperature, spin-crossover molecules appear as the ideal candidate for implementing spin devices at the molecular level.

Unique spin transition and wide thermal hysteresis loop for a cobalt(ii) compound with long alkyl chain

The cobalt(II) compounds with long alkyl chains, [Co(C12-terpy)(2)](BF(4))(2)·EtOH·0.5H(2)O(1·EtOH·0.5H(2)O) and [Co(C12-terpy)(2)](BF(4))(2) (1) was synthesized and characterized. The compound 1·EtOH·0.5H(2)O exhibits a "re-entrant spin crossover". The compound 1 exhibits the reentrant spin crossover and multi phase transitions with a wide thermal hysteresis loop.

Phase Transition Behavior of a Manganese(III) Spin-Crossover Complex Examined through Dielectric and Magnetic Responses

The octahedral manganese(III) complex, [MnIII(taa)] (H3taa = tris(1-(2-azolyl)-2-azabuten-4-yl)amine), has long been known as a rare example of (3d)4 spin-crossover system, having a steep spin-crossover phase transition at Tc = 48 K between a high-spin 5Eg and a low-spin 3T2g states. Since the high-spin state is obviously subject to the Jahn-Teller effect, the electronic states are coupled with lower-symmetry deformation modes lifting 5Eg orbital degeneracy in addition to a symmetrical breathing mode responsible for the spin crossover. This simultaneous involvement of two molecular bistabilities may afford a variety of condensed states (phases) due to competitive or cooperative interactions. The phase diagram and the phase transition behavior were thoroughly explored via several probes, including dielectric measurements, magnetization under pulsed high field, HF-EPR, and magnetic susceptibility measurements using high-pressure clamp cell. All the results were successfully interpreted in a unified fashion based on a four-state Ising-Potts model with mean-field approximation.

Combined, Modulation Enhanced X-ray Powder Diffraction and Raman Spectroscopic Study of Structural Transitions in the Spin Crossover Material [Fe(Htrz)2(trz)](BF4)

The structure of [Fe(Htrz)2(trz)]BF4 (1, Htrz =1,2,4-4-H-triazole, trz = 1,2,4-triazolate) at the low-spin (LS) and high-spin (HS) states and structural transitions between the two states were investigated by in situ high-resolution synchrotron X-ray powder diffraction (XRPD) combined with Raman spectroscopy using a modulation-enhanced technique. The crystal structures of the LS and HS states were determined. A 1D chain structure of 1 at both LS and HS states was proven, and the lattice expansion upon LS−HS transition was mainly caused by the elongation of the chain. The differences in the behavior of the spin transition observed by XRPD and Raman spectroscopy were explained by the local sensitivity of the two different techniques and also by the spatial propagation of spin crossover phase transition within the crystallite and the body of the grain. Moreover, we demonstrated that the two-dimensional correlation analyses facilitate (i) understanding the data obtained by combined techniques, (ii) clarifying...

Storing Spin-Crossover and LC Phase Transitions Information by Hybridizing Spin-Crossover Complexes with a Thermotropic Polymer Matrix – A Novel Case of Multiple Switching

We have prepared two new spin-crossover (SCO) Fe(II) complexes, [Fe(II)(ODT)3](p-tol)2(I) and [Fe(II)(HET)3](p-tol)2(II). And they were blended with a liquid crystalline (LC) poly(oxetane) (POx). Thermal and spin crossover behavior of the composites then were studied by using a superconducting quantum interference device (SQUID), differential scanning calorimetry (DSC) and extended X-ray absorption fine structure (EXAFS) spectroscopy. When composites carried higher than 20 wt.% of the Fe(II) complexes, they revealed a reversible spin crossover for the low spin (LS) ↔ high spin (HS) transition in addition to the thermal transitions, i.e., glass transition and isotropization, of the LC polymer matrix.

Numerical study on structure of metastable states in prussian blue analogues and their dynamical aspects

Prussian blue analogues (PBAs) show the functionality due to the interesting combination of spin-crossover phase transitions and magnetic ordering under photoirradiation. In order to understand the dynamics of the system, it is necessary to clarify the structure of metastable state. We study this structure from a viewpoint of degeneracy changes due to charge transfer between the cobalt and iron ions accompanying the spin-crossover transition. The existence of metastable ferrimagnetic state is found at low temperature which is lower than the hysteresis region of the spin-crossover phase transition. This fact well explains the existence of metastable structure at low temperature in experiments. Properties of PBAs in magnetic fields and dynamics in oscillating magnetic field are also studied.

Structures of Metastable States in Phase Transitions with a High-Spin Low-Spin Degree of Freedom

Difference of degeneracy of the low-spin (LS) and high-spin (HS) states causes interesting entropy effects on spin-crossover phase transitions and charge transfer phase transitions in materials composed of the spin-crossover atoms. Mechanisms of the spin-crossover (SC) phase transitions have been studied by using Wajnflasz model, where the degeneracy of the spin states (HS or LS) is taken into account and cooperative natures of the spin-crossover phase transitions have been well described. Recently, a charge transfer (CT) phase transition due to electron hopping between LS and HS sites has been studied by using a generalized Wajnflasz model. In the both systems of SC and CT, the systems have a high temperature structure (HT) and a low temperature structure (LT), and the change between them can be a smooth crossover or a discontinuous first order phase transition depending on the parameters of the systems. Although apparently the standard SC system and the CT system are very different, it is shown that both models are equivalent under a certain transformation of variables. In both systems, the structure of metastable state at low temperatures is a matter of interest. We study temperature dependence of fraction of HT systematically in a unified model, and find several structures of equilibrium and metastable states of the model as functions of system parameters. In particular, we find a reentrant type metastable branch of HT in a low temperature region, which would play an important role to study the photo-irradiated processes of related materials.

Observation of an Fe(II) Spin-Crossover in a Cesium Iron Hexacyanochromate

In this work, we observed a spin-crossover phenomenon in a cesium iron hexacyanochromate, which is a Prussian blue analogue. This compound showed a thermal phase transition with transition temperatures of 211 K (T1/2 downward arrow) and 238 K (T1/2 upward arrow) due to a spin-crossover on FeII sites. This spin-crossover phase transition is accompanied by a lattice contraction of 0.38 A, but maintains a face-centered cubic structure (F3m). This is the first observation of FeII spin-crossover in a series of Prussian blue analogues.

EXTERNAL PRESSURE AND LIGHT INFLUENCE ON INTERNAL PRESSURE IN A SPIN-CROSSOVER SOLID [Zn:Fe(ptz)6](BF4)2

The relation between the internal pressure during spin-crossover is compared to the chemical pressure induced by dilution with zinc. Further, the light of a specific LIESST (Light Induced Excited Spin State Trapping) wavelength is used to induce partial stabilisation of high-spin state and thus shift temperature of the spin-crossover towards lower values. The de-coupling of the spin-crossover and structural phase transition is discussed.

Pressure effect on temperature induced high-spin–low-spin phase transitions

The effect of hydrostatic pressure on the transition temperature and the hysteresis widths of first-order spin crossover phase transitions is considered in the frame of the mean field theory and on the basis of the scope of recent pressure experiments. Relevant parameters for a qualitative description of the behaviour of spin transition compounds under pressure are derived and analysed.

Effect of hydrostatic pressure on phase transitions in spin-crossover 1D systems

A model is developed to elucidate the role of hydrostatic pressure on the spin-crossover phase transitions in one-dimensional systems. The model involves long-range electron-deformational and short-range interactions. The conditions when pressure suppresses the thermal hysteresis are discussed. The pressure-induced system’s bistability is predicted. The qualitative explanation of experimental data is given.