Spin Transition Curves Research Articles

Role of the impurities in 2D spin crossover nanoparticle: Monte Carlo study

This work is devoted to the study of the impurities effect in 2D spin crossover nanosystem in frame work of an Ising-like model. Results were obtained by means of Monte Carlo modeling technique, based on the heat bath algorithm. It is shown, that the impurities change the system’s thermal hysteresis width and shift the spin transition curves (HS → LS). In this manner, impurities can act as an additional influence parameter on the transition curves. The considered effect is not significant, but nevertheless should be taken into account in the developing process of nanoscale devices based on spin crossover compounds.

Crystal Size Effect on the Spin-Crossover Behavior of {Fe(py)2[Pt(CN)4]} (py = Pyridine) Monitored by Raman Spectroscopy.

The spin-crossover phenomenon in nanomaterials has been the subject of exploratory investigations for stimuli-responsive switching properties at nanoscale. Using variable-temperature Raman spectroscopy, we investigated changes in the temperature-driven spin-transition property of {Fe(py)2[Pt(CN)4]} (py = pyridine) induced by a size reduction from a bulk polycrystalline powder to an ultrathin film (crystallite size, 15 nm). When the crystallite size was reduced, the spin-transition temperature shifted lower and the spin-transition curves became less steep. In the thin-film form, the residual high-spin fraction was found to be about 20%.

The Nano-Confinement Effect on the SCO Behavior of [Fe(NH2trz)3](NO3)2 1D Chains in MCM-41

A polymeric chain spin-crossover (SCO) compound [Fe(NH2trz)3](NO3)2 was embedded in the pores of mesoporous silica MCM-41, which yielded an M@S composite. The obtained material was characterized by infrared spectrometry, powder X-ray diffractometry, gas sorption studies, electron microscopy, and atomic absorption. The temperature-induced SCO behavior of the composite was monitored by temperature-variable magnetic measurement. The results indicate that the spin transition curve of the composite was evidently shifted to a higher temperature in comparison to bulk [Fe(NH2trz)3](NO3)2. The shift in the transition temperature for M@S [versus bulk [Fe(NH2trz)3](NO3)2] amounted to Tc↑ = 362 K [347 K] and Tc↓ = 327 K [314 K] (magnetic data). The significant difference in the SCO behavior of [Fe(NH2trz)3](NO3)2 when embedded in the MCM-41 matrix may be attributed to the strengthened cooperativity of compound S and the nano-confinement pressure of SiO2 walls.

Electro-vibrational Ising-type model for spin crossover in binuclear molecules

A spin crossover in binuclear molecule is investigated using an Ising-type model, including the intramolecular vibrations. The spin transition curve of [Fe(bt)(NCS)2]2bpym complex which presents a two-step transition plateau is well described by the above model. We show that the equilibrium low-spin/high-spin temperature $$ T_{\text{eq}} $$ is independent of the vibration frequencies of the intermediate state (HS–LS or LS–HS) $$ \omega_{\text{LHS}} $$ . In addition, a large values of LS–LS state vibration frequencies $$ \omega_{\text{LLS}} $$ lead to an evident two-step transition plateau.

A novel fabrication of [Fe(HB(pz)3)2]@MIL-101 hybrid material via diffusion and the lower temperature shift on its spin transition behavior

[Fe(HB(pz)3)2], a well-studied spin-crossover (SCO) complex, has been entrapped in NH2-MIL-101(Al) metal–organic framework (MOF) via gas diffusion method. [Fe(HB(pz)3)2] would sublimate when temperature exceeds 100 °C, especially at low pressure. Thus, in the selected condition, [Fe(HB(pz)3)2] molecules can be sublimated to a gas phase and diffused into the pores of NH2-MIL-101(Al), which yield [Fe(HB(pz)3)2]@MIL-101 hybrid materials. The identity of composites, with a loading of iron complex at ~ 8 wt%, was characterized by X-ray diffraction and spectroscopic attestation. The hybrid materials demonstrate a gentle spin transition curve from 300 to 400 K, which is different from the SCO behavior of [Fe(HB(pz)3)2]. And for the pure [Fe(HB(pz)3)2] sample at 400 K, only 60% of Fe(II) is in HS, while in hybrid samples, HS state is complete. This interesting phenomenon might indicate that the spin transition in the hybrid sample can be triggered at lower temperature. The composite samples were thoroughly studied by X-ray diffraction, IR spectroscopy, atom absorption spectroscopy, nitrogen physisorption, and magnetic measurements. Thereby, a novel MOF-based material with isolated SCO units is proposed, which demonstrate a salient ‘matrix-effect’ on spin-crossover behavior of [Fe(HB(pz)3)2].

Evolution of cooperativity in the spin transition of an iron(II) complex on a graphite surface

Cooperative effects determine the spin-state bistability of spin-crossover molecules (SCMs). Herein, the ultimate scale limit at which cooperative spin switching becomes effective is investigated in a complex [Fe(H2B(pz)2)2(bipy)] deposited on a highly oriented pyrolytic graphite surface, using x-ray absorption spectroscopy. This system exhibits a complete thermal- and light-induced spin transition at thicknesses ranging from submonolayers to multilayers. On increasing the coverage from 0.35(4) to 10(1) monolayers, the width of the temperature-induced spin transition curve narrows significantly, evidencing the buildup of cooperative effects. While the molecules at the submonolayers exhibit an apparent anticooperative behavior, the multilayers starting from a double-layer exhibit a distinctly cooperative spin switching, with a free-molecule-like behavior indicated at around a monolayer. These observations will serve as useful guidelines in designing SCM-based devices.

Correlating Properties in Iron(III) Complexes: A DFT Description of Structure, Redox Potential and Spin Crossover Phenomena

Calculations of structure, redox potential and spin transition energies were performed using DFT approximations for a series of [Fe(bztpen)OR]2+/+ type FeIII/FeII systems (R=Me, Et, nPr, nBu), which have a temperature dependent spin crossover behavior. These compounds exhibit changes in redox and magnetic properties, related to structural variations quite important for their possible applications among which signal generator materials stand out. Functionals B3LYP, ωB97X−D and TPSS along with PCM solvation model were evaluated for redox potential, whereas for spin crossover the TPSSh functional was also included. The multireference character of these compounds was tested as well. Calculations were compared to experimental measurements, and ωB97X−D proved able to accurately describe the geometries observed in solid state for the low spin (LS) and high spin (HS) states; moreover, it had the best correlation between calculated and experimental redox potential values. However, in the description of the spin transition energies the TPSS functional is needed to correctly describe the LS state as the observed ground state in the complexes at low temperature, which allows to calculate proper spin transition curves as a function of temperature. From these results, we obtained suitable approximations for an accurate description of redox potential and magnetic properties for the FeIII coordination compounds, which can be extended to model similar systems.

Reprint of “Surface-environment effects in spin crossover solids”

Abstract The impact of surface effects on thermal induced spin crossover phenomenon is a subject of a broad and current interest. Using the modified Ising-like model of spin crossover solids with the ligand field as function of the molecule’ positions and random component on surface by means of Metropolis Monte Carlo algorithm the thermal spin transition curves were calculated. The analysis of spin configuration during transition gives a general idea about contribution of molecules from the surface and inside the lattice into resulting magnetization of the systems. The behavior of hysteresis loop for various surface coupling and fluctuations strength has been described.

Surface-environment effects in spin crossover solids

The impact of surface effects on thermal induced spin crossover phenomenon is a subject of a broad and current interest. Using the modified Ising-like model of spin crossover solids with the ligand field as function of the molecule’ positions and random component on surface by means of Metropolis Monte Carlo algorithm the thermal spin transition curves were calculated. The analysis of spin configuration during transition gives a general idea about contribution of molecules from the surface and inside the lattice into resulting magnetization of the systems. The behavior of hysteresis loop for various surface coupling and fluctuations strength has been described.

Two-step spin transition in a 1D Fe(II) 1,2,4-triazole chain compound.

A thermochromic 1D spin crossover coordination (SCO) polymer [Fe(βAlatrz)3](BF4)2⋅2 H2O (1⋅2 H2O), whose precursor βAlatrz, (1,2,4-triazol-4-yl-propionate) has been tailored from a β-amino acid ester is investigated in detail by a set of superconducting quantum interference device (SQUID), (57)Fe Mössbauer, differential scanning calorimetry, infrared, and Raman measurements. An hysteretic abrupt two-step spin crossover (T1/2(↓) = 230 K and T1/2(↑) = 235 K, and T1/2(↓) = 172 K and T1/2(↑) = 188 K, respectively) is registered for the first time for a 1,2,4-triazole-based Fe(II) 1D coordination polymer. The two-step SCO configuration is observed in a 1:2 ratio of low-spin/high-spin in the intermediate phase for a 1D chain. The origin of the stepwise transition was attributed to a distribution of chains of different lengths in 1⋅2 H2O after First Order Reversal Curves (FORC) analyses. A detailed DFT analysis allowed us to propose the normal mode assignment of the Raman peaks in the low-spin and high-spin states of 1⋅2 H2O. Vibrational spectra of 1⋅2 H2O reveal that the BF4(-) anions and water molecules play no significant role on the vibrational properties of the [Fe(βAlatrz)3](2+) polymeric chains, although non-coordinated water molecules have a dramatic influence on the emergence of a step in the spin transition curve. The dehydrated material [Fe(βAlatrz)3](BF4)2 (1) reveals indeed a significantly different magnetic behavior with a one-step SCO which was also investigated.

Role of surface vibrational properties on cooperative phenomena in spin-crossover nanomaterials

The influence of surface/interface on the lattice dynamics of spin crossover nanoparticles has been investigated by a spring-ball model solved by Monte Carlo methods. The bond cohesion energy of the model has been extracted from M\"ossbauer spectroscopy measurements performed on the model compound ${\mathrm{Ni}}_{3}[\mathrm{Fe}{(\mathrm{CN})}_{6}]$. We show that the coupling between bulk and surface vibrational properties, which drastically affects the mechanical properties of the whole particle below a characteristic size, has a major impact on the phase stability of the particles. In the case of free surfaces, the Debye temperature decreases with the size and the first-order nature of the spin transition disappears. On the other hand, a hardening of the surface bonds leads to increasing particle stiffness with the size reduction. In this case, a persistence of the hysteretic behavior in the spin transition curve is also predicted in good agreement with previous theoretical and experimental results.

Role of open boundary conditions on the hysteretic behaviour of one-dimensional spin crossover nanoparticles

In order to explain clearly the role of the open boundary conditions (OBCs) on phase transition in one dimensional system, we consider an Ising model with both short-range (J) and long-range (G) interactions, which has allowed us to study the cooperative nature of spin-crossover (SCO) materials at the nanometer scale. At this end, we developed a transfer-matrix method for one-dimensional (1D) SCO system with free boundary conditions, and we give numerical evidences for how the thermal spin transition curves vary as a function of the physical parameters (J, G) or an applied pressure. Moreover for OBCs case, we have derived the bulk, surface and finite-size contributions to the free energy and we have investigated the variation of these energies as function of J and system size. We have found that the surface free energy behaves like J⟨σ⟩2, where ⟨σ⟩ is the average magnetization per site. Since the properties of the nanometric scale are dramatically influenced by the system's size (N), our analytical outcomes for the size dependence represent a step to achieve new characteristic of the future devices and also a way to find various novel properties which are absent in the bulk materials.

Vibration-driven thermal transition in binuclear spin crossover complexes

An Ising-like model including the effect of intramolecular vibrations is proposed to investigate Fe(II) binuclear spin crossover complexes, especially in the situation where it allows the assessment of the conditions under which cooperative exchange interactions are weak enough to favor a vibration-driven mechanism for the thermal transition. In particular, two spin states (LS-LS, LS-HS or HS-HS where LS and HS stand for low-spin and high-spin respectively) must be quasi-degenerate. Whatever the equi-energy scenario, the effect of molecular vibrations on the groundstate appears in the low temperature region, where a deviation from the low frequency approximation of the standard Ising-like model is observed. Thirteen quasi-equi-energy situations lead to three possible vibronic groundstates at T = 0 K, from which the thermal evolution of the HS fraction presents a similar trend as mononuclear systems. However, binuclear systems permit to generate a HS-LS vibronic groundstate. For this latter case the application of pressure, known to drastically affect the spin transition, is suggested to reveal a two-step behavior according to a LS-LS, HS-LS HS-HS scheme. Intramolecular coupling between the two metallic centres and intermolecular cooperativity few influence the spin transition curves due to a difference of the temperature ranges between the effects of molecular vibrations and spin crossover phenomenon.

Spin Transition Sensors Based on β-Amino-Acid 1,2,4-Triazole Derivative

A β-aminoacid ester was successfully derivatized to yield to 4H-1,2-4-triazol-4-yl-propionate (βAlatrz) which served as a neutral bidentate ligand in the 1D coordination polymer [Fe(βAlatrz)3](CF3SO3)2·0.5H2O (1·0.5H2O). The temperature dependence of the high-spin molar fraction derived from 57Fe Mossbauer spectroscopy recorded on cooling below room temperature reveals an exceptionally abrupt single step transition between high-spin and low-spin states with a hysteresis loop of width 4 K (Tc↑ = 232 K and Tc↓ = 228 K) in agreement with magnetic susceptibility measurements. The material presents striking reversible thermochromism from white, at room temperature, to pink on quench cooling to liquid nitrogen, and acts as an alert towards temperature variations. The phase transition is of first order, as determined by differential scanning calorimetry, with transition temperatures matching the ones determined by SQUID and Mössbauer spectroscopy. The freshly prepared sample of 1·0.5H2O, dried in air, was subjected to annealing at 390 K, and the obtained white compound [Fe(βAlatrz)3](CF3SO3)2 (1) was found to exhibit a similar spin transition curve however much temperature was increased by (Tc↑ = 252 K and Tc↓ = 248 K). The removal of lattice water molecules from 1·0.5H2O is not accompanied by a change of the morphology and of the space group, and the chain character is preserved. However, an internal pressure effect stabilizing the low-spin state is evidenced.

Prediction of the Spin Transition Temperature in FeII One-Dimensional Coordination Polymers: an Anion Based Database

One-dimensional (1D) coordination polymers of formula [Fe(NH(2)trz)(3)]A.nH(2)O, {A = TiF(6)(2-), n = 0.5 (1) and n = 1 (2); A = ZrF(6)(2-), n = 0.5 (3) and n = 0 (4); A = SnF(6)(2-), n = 0.5 (5) and n = 1 (6); A = TaF(7)(2-), n = 3 (7) and n = 2.5 (8); A = GeF(6)(2-), n = 1 (9) and n = 0.5 (10), NH(2)trz = 4-amino-1,2,4-triazole} have been synthesized, fully characterized, and their spin crossover behavior carefully studied by SQUID magnetometry, Mossbauer spectroscopy, and differential scanning calorimetry. These materials display an abrupt and hysteretic spin transition around 200 K on cooling, as well as a reversible thermochromic effect. Accurate spin transition curves were derived by (57)Fe Mossbauer spectroscopy considering the corrected f factors for the high-spin and low-spin states determined employing the Debye model. The unusual hysteresis width of 3 (28 K), was attributed to a dense hydrogen bonding network involving the ZrF(6)(2-) counteranion and the 1D chains, an organization which is also revealed in [Cu(NH(2)trz)(3)]ZrF(6).H(2)O (11). Trinuclear spin crossover compounds of formula [Fe(3)(NH(2)trz)(10)(H(2)O)(2)](SbF(6))(6).S {S = 1.5CH(3)OH (12), 0.5C(2)H(5)OH (13)} were also obtained. A structural property relationship was derived between the volume of the inserted counteranion and the transition temperature T(1/2) of the 1D chains. Two linear size regimes were identified for monovalent anions (0.04 <or= V (nm(3)) <or= 0.09) and for divalent anions (above V >or= 0.11 nm(3)) with saturation around T(1/2) = 200 K. These characteristics allowed us to derive an anion based database that is of interest for the prediction of the transition temperature of such functional switchable materials. Diffuse reflectivity measurements under hydrostatic pressure for 3,4 combined with calorimetric data allow an estimation of the electrostatic pressure between cationic chains and counteranions in the crystal lattice of these materials. The chain length distribution that ranges between 1 and 4 nm was also derived.

Pressure-induced hysteresis in the high spin low spin transition in bis(2,4-bis(pyridin-2-yl)thiazole) iron(II) tetrafluoroborate

Studies of the spin transition behavior of the mononuclear compound[Fe(pythiaz)2](BF4)2 have been carried out under hydrostatic pressures up to 9.13 kbar in the 5–300 Ktemperature range. Under ambient pressure this compound exhibits an approximatelyhalf-step (incomplete) HS LS transition with T1/2 = 146 K without any thermal hysteresis. At pressures up to 4.5 kbarthe behavior remains similar but with an upward displacement ofT1/2 and a slight decrease in the residual high spin fraction at lowT. Application of higher pressures resulted in an almost complete two-step spin transitionwith several unusual pressure effects. Along with the expected pressure dependence ofT1/2 the surprising appearance of hysteresis in the spin transition curves was observed. It issuggested that the likely origin of this unprecedented behavior is a pressure-inducedstructural change.

Mössbauer spectroscopic study of the thermal spin crossover in [Fe(II)(isoxazole) 6](ClO 4) 2

The 57Fe Mössbauer spectroscopy of mononuclear [Fe(II)(isoxazole) 6](ClO 4) 2 has been studied to reveal the thermal spin crossover of Fe(II) between low-spin ( S=0) and high-spin ( S=2) states. Temperature-dependent spin transition curves have been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures between 84 and 270 K during a cooling and heating cycle. This compound exhibits an unusual temperature-dependent spin transition behaviour with T C(↓)=223 and T C(↑)=213 K occurring in the reverse order in comparison to those observed in SQUID observation and many other spin transition compounds. The compound has three high-spin Fe(II) sites at the highest temperature of study of which two undergo spin transitions. The compound seems to undergo a structural phase transition around the spin transition temperature, which plays a significant role in the spin crossover behaviour as well as the magnetic properties of the compound at temperatures below T C. The present study reveals an increase in high-spin fraction upon heating in the temperature range below T C, and an explanation is provided.

Rapid Cooling Experiments and Use of an Anionic Nuclear Probe to Sense the Spin Transition of the 1D Coordination Polymers [Fe(NH2trz)3]SnF6⋅n H2O (NH2trz=4‐amino‐1,2,4‐triazole)

[Fe(NH2trz)3]SnF6n x H2O (NH(2)trz=4-amino-1,2,4-triazole; n=1 (1), n=0.5 (2)) are new 1D spin-crossover coordination polymers. Compound 2 exhibits an incomplete spin transition centred at around 210 K with a thermal hysteresis loop approximately 16 K wide. The spin transition of 2 was detected by the Mössbauer resonance of the 119Sn atom in the SnF6 (2-) anion primarily on the basis of the evolution of its local distortion. Rapid-cooling 57Fe Mössbauer and superconducting quantum interference device experiments allow dramatic widening of the hysteresis width of 2 from 16 K up to 82 K and also shift the spin-transition curve into the room temperature region. This unusual behaviour of quenched samples on warming is attributed to activation of the molecular motion of the anions from a frozen distorted form towards a regular form at temperatures well above approximately 210 K. Potential applications of this new family of materials are discussed.

Study of thermal spin crossover in[Fe(II)(isoxazole)6](BF4)2 with Mössbauer spectroscopy

57Fe Mössbauer spectroscopyof the mononuclear [Fe(II)(isoxazole)6](BF4)2 compound has been studied to reveal the thermal spin crossover of Fe(II) between low-spin(S = 0) andhigh-spin (S = 2) states. A temperature-dependent spin transition curve has been constructed with theleast-square fitted data obtained from the Mössbauer spectra measured at varioustemperatures in the 240–60 K range during the cooling and heating cycle. The compoundexhibits a temperature-dependent two-step spin transition phenomenon withTSCO (step1) = 92and TSCO (step 2) = 191 K. The compound has three high-spin Fe(II) sites at the highest temperatureof study; among them, two have slightly different coordination environments.These two Fe(II) sites are found to undergo a spin transition, while the thirdFe(II) site retains the high-spin state over the whole temperature range. Possiblereasons for the formation of the two steps in the spin transition curve are discussed.The observations made from the present study are in complete agreementwith those envisaged from earlier magnetic and structural studies made on[Fe(II)(isoxazole)6](BF4)2, but highlights the nature of the spin crossover mechanism.

A novel spin transition curve in [tris(2-picolylamine)Fe(II)]Cl2allyl solvate

A novel spin transition curve in [tris(2-picolylamine)Fe(II)]Cl₂allyl solvate